[feature] spate-gcs: a Google Cloud Storage backfill source

[MarcusKainth]

What kind

A new connector (source, sink, or deserializer)

Which components

S3 source (spate-s3)

What you are trying to do, and what stops you

Part of #749. A backfill from Google Cloud Storage has no source: spate-s3 enables only the object_store aws feature (crates/spate-s3/Cargo.toml:27), so a gs:// URL fails when the store is built.

spate-gcs is a store crate over the backfill engine extracted in #750, with the gcp client built by the shared builder crate from #685. It carries:

• GcsSource and GcsSourceConfig, with the same fields as S3SourceConfig and gs:// URLs;
• the spate_gcs_source_* metric namespace;
• a gcs feature on the spate facade;
• a docs page under docs/user-guide/04-connectors/sources/.

Client hardening. spate-s3 builds its S3 client from the environment and applies hardened client defaults (crates/spate-s3/src/source.rs:403). The gcp builder also reads the environment, so the GCS client needs the same review of which variables it honours and which defaults it hardens, with tests.

Rejected credentials. spate-s3 recognises a listing rejected with 401 or 403 by matching the S3 client's error text (crates/spate-s3/src/error.rs:68), because object_store reports it as Generic. How the gcp client reports a rejected listing and a rejected read is unverified. A failing test against a local server pins both before the classification is written.

Split identity. Each store crate passes its own domain tag and fingerprint prefix to the engine, so a GCS job's persisted ids are never confused with an S3 job's.

This pull request adds the ADR for the one-crate-per-store layout, since it is the first time two store crates use the engine.

Blocked by #750 and by #685.

Line numbers are on a58ebf94.

Semver impact, as best you can tell

Additive — nothing existing changes

Does this touch any of these?

• INV-1 — delivery semantics, or anything that could commit a watermark past unacknowledged data
• INV-2 — the source poll loop, which must never block on a channel send
• INV-3 — the checkpoint tracker, which is synchronous and free of async runtimes
• INV-5 — the sink intake path, which must not await outside its select!
• INV-6 — adding a connector or 0.x dependency type to a public signature in spate-core
• INV-8, INV-9 or INV-10 — metric naming, cardinality, or gauge ownership

The crate registers a new spate_gcs_source_* namespace with the same families as spate_s3_source_*.

Would you want to implement it?

Yes, if the design is agreed