feat: Add externalId to MergeRequest API

- DMD-1081
- Expose the MergeRequest::$externalId through the create and update
  merge request endpoints as an optional string parameter.
- Update response provider to include externalId in all MR responses,
  and update Apiary documentation accordingly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: martinsifra

apiary.apib

@@ -9901,6 +9901,7 @@ other buckets (production buckets or buckets belongs the other development branc
     + branchIntoId (required, number) - ID of the branch to merge into (default branch)
     + title (required, string) - title of the merge request
     + description (optional, string) - description of the merge request
+    + externalId (optional, string) - identifier of the merge request in an external system
 
 + Request (application/json)
 
@@ -9915,6 +9916,7 @@ other buckets (production buckets or buckets belongs the other development branc
                 "branchIntoId": 123,
                 "title": "Refactor DB extractors",
                 "description": "DB exctractor have new columns now",
+                "externalId": "ticket-123"
             }
 
 + Response 201 (application/json)
@@ -9939,6 +9941,7 @@ other buckets (production buckets or buckets belongs the other development branc
                   "mergerId": null,
                   "mergerName": ""
             },
+            "externalId": "ticket-123",
             "approvals": [
             {
                   "approverId": 123,
@@ -9955,6 +9958,7 @@ other buckets (production buckets or buckets belongs the other development branc
 + Attributes
     + title (required, string) - title of the merge request
     + description (optional, string) - description of the merge request
+    + externalId (optional, string) - identifier of the merge request in an external system
 
 + Request (application/json)
 
@@ -9967,6 +9971,7 @@ other buckets (production buckets or buckets belongs the other development branc
             {
                 "title": "New title",
                 "description": "New desc",
+                "externalId": "ticket-123"
             }
 
 + Response 200 (application/json)
@@ -9991,6 +9996,7 @@ other buckets (production buckets or buckets belongs the other development branc
                   "mergerId": null,
                   "mergerName": ""
             },
+            "externalId": "ticket-123",
             "approvals": [
             {
                   "approverId": 123,
@@ -10034,6 +10040,7 @@ List all merge requests available in the project defined by token.
                       "mergerId": null,
                       "mergerName": ""
                 },
+                "externalId": "ticket-123",
                 "approvals": [
                 {
                       "approverId": 123,
@@ -10097,6 +10104,7 @@ Detail of Merge request also contains `content` of changed objects.
                   "mergerId": null,
                   "mergerName": ""
             },
+            "externalId": "ticket-123",
             "approvals": [
             {
                   "approverId": 123,
@@ -10157,6 +10165,7 @@ Will move Merge request state `development` -> `in_review`. If branch has an ope
                   "mergerId": null,
                   "mergerName": ""
             },
+            "externalId": "ticket-123",
             "approvals": [
             {
                   "approverId": 123,
@@ -10208,7 +10217,7 @@ Each project has defined required number of approvals. If the last approval arri
                         "lastVersionIdentifier": "01H5M1B47BX3W4GM4CDH40J330"
                     }
                 ]
-            }
+            },
             "branches": {
                   "branchFromId": 124,
                   "branchIntoId": 123
@@ -10218,6 +10227,7 @@ Each project has defined required number of approvals. If the last approval arri
                   "mergerId": null,
                   "mergerName": ""
             },
+            "externalId": "ticket-123",
             "approvals": [
             {
                   "approverId": 123,
@@ -10299,7 +10309,7 @@ Requesting changes will remove all approvals and move the Merge request to `deve
                         "lastVersionIdentifier": "01H5M1B47BX3W4GM4CDH40J330"
                     }
                 ]
-            }
+            },
             "branches": {
                   "branchFromId": 124,
                   "branchIntoId": 123
@@ -10309,6 +10319,7 @@ Requesting changes will remove all approvals and move the Merge request to `deve
                   "mergerId": null,
                   "mergerName": ""
             },
+            "externalId": "ticket-123",
             "approvals": [
             {
                   "approverId": 123,