[AMORO-4240][AMS] Support staged table creation in REST catalog

[johntomcat7408-cmyk]

Summary

This is the second PR for #4240.

• Honor CreateTableRequest.stageCreate() for internal Iceberg REST catalogs.
• Return staged metadata without writing files to the table location or registering the table in AMS.
• Complete table creation when the client sends the following create commit.
• Keep direct table creation and normal metadata commits unchanged.
• Add an integration test for the staged-create transaction.

Why

Trino expects the target table location to remain empty between staged creation and the final commit. AMS previously wrote the initial metadata file during the staged request, so Trino rejected the location as non-empty.

Verification

• mvn -ntp validate -pl amoro-ams -am -DskipTests -Pskip-dashboard-build
• `mvn -ntp test -pl amoro-ams -am