batch processing w/ exponential backoff

[bdehamer]

Introduces a new hard limit of 2500 subjects which can be attested with a single invocation of the action.

Subjects are processed in batches of 50 with an exponential delay added after the initial batch is processed:

• Batch 2: 7.5 seconds (150ms per attestation)
• Batch 3: 15 seconds (300ms per attestation)
• Batch 4: 30 seconds (600ms per attestation)
• Batches 5-50: 60 seconds (1200ms per attestation)

[carlocab]

Thanks! Would be great if this could be pushed downstream to https://github.com/actions/attest-build-provenance