Move client auth APIs to a separate module

This makes it possible to write new client auth implementations without
bringing in the entire client runtime as a dependency.

Author: rhernandez35

client/client-auth-api/build.gradle.kts

@@ -0,0 +1,14 @@
+plugins {
+    id("smithy-java.module-conventions")
+}
+
+description = "This module provides the client auth APIs"
+
+extra["displayName"] = "Smithy :: Java :: Client :: Auth API"
+extra["moduleName"] = "software.amazon.smithy.java.client.core.auth"
+
+dependencies {
+    api(project(":auth-api"))
+    api(project(":core"))
+    implementation(libs.smithy.model)
+}