Add GCS sink destination support to define_sink_pipe

[sandshoes]

Summary

• define_sink_pipe currently only supports Kafka and S3 destinations. Adds GCS as a third sink destination, matching the Forward CLI's gcs_hmac export service, for https://linear.app/tinybird/issue/PROTM-2262/python-sdk-add-gcs-sink-destination-support
• New GCSSinkConfig mirrors S3SinkConfig's field set (bucket_uri, file_template, format, schedule, strategy, compression) but references a GCSConnectionDefinition instead of S3ConnectionDefinition.
• Since GCS and S3 sinks share an identical EXPORT_* directive shape in .pipe datafiles, the generator emits an explicit EXPORT_SERVICE gcs_hmac directive for GCS sinks so the migration parser can disambiguate them from S3 sinks on reverse-parse (S3 remains the default when EXPORT_SERVICE is omitted, preserving existing behavior/datafiles).
• Updated the migrate runner's sink/connection compatibility check, which previously compared pipe.sink.service directly against the connection's _connectionType — this would have incorrectly rejected a valid gcs_hmac sink pointed at a gcs connection. It now maps gcs_hmac → gcs before comparing.
• Also wired SinkGCSModel through migrate/emit_ts.py for TypeScript re-emission parity with S3.

Changes

• src/tinybird_sdk/schema/pipe.py: GCSSinkConfig dataclass, SinkConfig union, _normalize_sink_config GCS branch.
• src/tinybird_sdk/generator/pipe.py: emit EXPORT_SERVICE gcs_hmac + shared export directives for GCSSinkConfig.
• src/tinybird_sdk/migrate/types.py: SinkGCSModel.
• src/tinybird_sdk/migrate/parse_pipe.py: accept gcs_hmac as an EXPORT_SERVICE value; build SinkGCSModel when explicitly set, default to s3 otherwise (unchanged behavior for existing datafiles).
• src/tinybird_sdk/migrate/runner.py: fix sink/connection type compatibility check for gcs_hmac.
• src/tinybird_sdk/migrate/emit_ts.py, schema/__init__.py, migrate/__init__.py: wiring/exports.
• README.md, CHANGELOG.md: documented the new sink option.
• tests/test_gcs_sink_destination.py (new, 8 tests): schema validation, datafile generation, parse round-trip (explicit service + default-to-s3 fallback + unsupported service rejection), runner compatibility (accept/reject), and TS emission.

Checklist

• CI is green (lint, typecheck, test)
• pre-commit run --all-files passes locally (gitleaks step fails in this sandbox on an unrelated SSL/network error fetching its pre-commit environment, not a code issue)
• Tests were added or updated when behavior changed
• Public API / typing changes were reviewed
• Documentation was updated (README.md)
• Breaking changes are clearly documented (none — purely additive; default inference for ambiguous existing sinks is unchanged)
• CHANGELOG.md was updated when user-facing behavior changed