Record the gRPC target on cassette interactions

[davinat0r]

Adds a target field to cassette interactions, capturing the gRPC server address (host:port) each request was recorded against.

RecordingChannel/AsyncRecordingChannel set it automatically from their target constructor argument; a pre-seeded Cassette.target takes precedence (first-write-wins) if already set.

Also adds a TargetMatcher for projects that record against multiple hosts and need playback to distinguish interactions
by target.

This is purely additive and backward-compatible — cassette YAMLs recorded before this change simply omit target, load unchanged, and default matching (MethodMatcher) is unaffected.

Checklist

Before Review

• Code has been manually reviewed (if AI-generated)
• make format and make typecheck pass
• PR title is suitable for release notes

Before Merge

• Tests added/updated (100% coverage required)
• Documentation updated (if adding new features)