Add multiauth keychains

[wlynch]

Adds support for multiauth keychaings for chainguard authentication. We primarily want this to allow authentication for multiple hosts (e.g. staging and test instances). This creates a new URL-based schema for specifying Chainguard identities of the form chainguard://[email protected]?iss=issuer.enforce.dev that encodes all of the identity information we need into a string that we can pass into the CHAINGUARD_IDENTITY environment var.

Backwards compatibility was kept with the existing env var format.