@actions/attest: Support multi-subject attestations

[bdehamer]

Updates the attest and attestProvenance functions to accept an array of subjects which can be added to a single attestation.

Previously, these functions accepted a single subject name and digest -- allowing the consumer to create an attested intoto statement for just a single subject. This change deprecates the subjectName/subjectDigest params in favor of a new subjects option which accepts an array of subjects.

[kommendorkapten]

Looks good!

Failing test on macOS seems unrelated:

Summary of all failing tests
FAIL packages/exec/__tests__/exec.test.ts (6.996 s)
  ● @actions/exec › Handles child process holding streams open

Is this an intermittent failure or does it fail on main too?