Add pscale branch extensions list

[no-itsbackpack]

Lists extensions available on a Postgres branch's cluster image (GET …/extensions). This is the catalog, not CREATE EXTENSION state, and there is no public enable API.

pscale branch extensions list mydb main --org myorg
pscale branch extensions mydb main --org myorg

Preload / session libraries are still configured with pscale branch resize --parameters.

[cursor]

Risk: low. Approved: Cursor Bugbot completed successfully with no findings requiring human review, and Cursor Security Agent was not present on this PR. No reviewers were assigned.

  

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