Initial multi-backend objectstorage client implementation

This MR introduces a gocloud.dev/blob-backed object storage client.

Based largely on this Claude generated plan: gitlab-org/quality/quality-engineering/team-tasks#4456 (comment 3466169269)

This MR:

• Builds out a client model similar to our Postgres and Redis packages. (Config, Client, NewFromConfig)
• Builds out a pluggable backend Provider pattern
• Adds an initial S3 Provider implementation (OpenBucket)

A couple notable decisions:

• Provider packages are built out into their own sub-packages. This is similar to the pattern gocloud.dev/blob uses; avoiding pulling in all backends as dependencies.
• I added an in-memory S3 server (gofakes3 over httptest), mirroring the redistest helper, so package can test S3-backed code without a real bucket
• Added an in-memory provider using "gocloud.dev/blob/memblob"
  • Was helpful while building out he provider model, but it's purpose might partially be covered by the above mock server.

Not in this MR:

• Objectstorage configuration proto messages (To live in labkit-spec)
• ResolveFromProto / methods for pulling config from attached secrets / known config locations.
• Additional S3 authentication methods
• Azure and GCS backends.
• Logging, metrics, and tracing wired through each provider backend SDK