feat(cawg_identity): Define `CredentialHolder` trait #4080

Job	Run time
Doc tests (requires nightly Rust) (windows-latest)	6m 21s
Default features build	16s
Doc tests (requires nightly Rust) (macos-latest)	2m 28s
Unit tests (windows-latest, stable)	4m 7s
Doc tests (requires nightly Rust) (ubuntu-latest)	1m 58s
Unit tests (windows-latest, 1.81.0)	3m 47s
Unit tests (c2patool) (windows-latest, stable)	3m 55s
Unit tests (macos-latest, stable)	1m 25s
Unit tests (c2patool) (macos-latest, stable)	58s
Unit tests (macos-latest, 1.81.0)	1m 19s
Unit tests (c2patool) (ubuntu-latest, stable)	1m 1s
Unit tests (ubuntu-latest, stable)	1m 8s
Unit tests (ubuntu-latest, 1.81.0)	1m 13s
Enforce Rust code format	13s
License / vulnerability audit (advisories)	27s
License / vulnerability audit (bans licenses sources)	25s
Clippy	39s
Preflight docs.rs build	2m 5s
Unit tests with minimum versions of direct dependencies (windows-latest)	3m 39s
Unit tests with minimum versions of direct dependencies (macos-latest)	1m 26s
Unit tests with minimum versions of direct dependencies (ubuntu-latest)	1m 3s
Unit tests (WASM)	2m 29s
Unit tests (aarch64-unknown-linux-gnu, stable)	4m 22s
Unit tests (aarch64-unknown-linux-gnu, 1.81.0)	4m 25s
Check for unused dependencies	2m 24s
	53m 33s

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