Add Kibana Streams API (/api/streams, /api/streams/_status) to diagnostics

[lucabelluccini]

Summary

• Adds kibana_streams (GET /api/streams) to kibana-rest.yml so the diagnostic collects Kibana Streams configuration.
• Adds streams_status (GET /_streams/status) to elastic-rest.yml so the diagnostic collects stream enablement status directly from Elasticsearch. Kibana's own GET /api/streams/_status is a thin proxy over this Elasticsearch endpoint and was rejected by at least one Kibana build ("exists but is not available with the current configuration"), so we query Elasticsearch directly instead.
• Both are version-gated to >= 8.19.0 < 9.0.0 || >= 9.1.0, matching where these APIs actually exist (confirmed against the 8.19.0/9.0.0/9.1.0 release tags for both Kibana and Elasticsearch — the APIs are present in the 8.19.0 and 9.1.0 trains, but not in 9.0.x).
• Neither endpoint supports pagination.

Test plan

• ./gradlew compileJava succeeds
• ./gradlew test --tests "co.elastic.support.rest.*" passes (existing YAML-loading tests still pass with the new entries)
• Built the dist zip and ran kibana-api diagnostics against a local Kibana 9.5.0 instance — kibana_streams.json collected real stream data successfully
• Manually run diagnostics against a Kibana/Elasticsearch 9.1.0+ (or 8.19.0+) instance and confirm streams_status.json is produced
• Confirm the entries are correctly skipped on 9.0.x

[pickypg]

@lucabelluccini I'll wait until you can confirm those last two, then I can merge this.

[lucabelluccini]

Hello @flash1293 / @3kt - May I ask if those APIs make sense and if we should gate them on a 9.2 or even 9.4 version instead?
Can those APIs leak PII/data?

[lucabelluccini]

Hello @pickypg - I need a review by Joe/Alexis - Let's hold please

Tests run against 9.5.0-SNAPSHOT

Elasticsearch (streams_status.json, from GET /_streams/status):
{"logs":{"enabled":false},"logs.ecs":{"enabled":true},"logs.otel":{"enabled":true}}

Kibana (kibana_streams.json, from GET /api/streams):
{"streams":[{"type":"wired","name":"logs.ecs",...},{"type":"wired","name":"logs.otel",...},{"type":"classic","name":"logs-elastic_agent-default",...}, ...]}

[flash1293]

LGTM, /api/streams is not super light, but since the number of streams is bounded by what Elasticsearch can handle (a couple thousand at max), it shouldn't cause crazy memory pressure.