Add new type rfc3161 for RFC 3161 timestamp responses (#324)

Makefile

cmd/rekor-cli/app/pflags.go

cmd/rekor-cli/app/upload.go

cmd/rekor-server/app/serve.go

openapi.yaml

pkg/generated/models/proposed_entry.go

pkg/generated/models/rfc3161.go

pkg/generated/models/rfc3161_schema.go

pkg/generated/models/rfc3161_v001_schema.go

pkg/generated/restapi/embedded_spec.go

pkg/types/rfc3161/rfc3161.go

pkg/types/rfc3161/rfc3161_schema.json

pkg/types/rfc3161/rfc3161_test.go

pkg/types/rfc3161/v0.0.1/entry.go

pkg/types/rfc3161/v0.0.1/entry_test.go

pkg/types/rfc3161/v0.0.1/rfc3161_v0_0_1_schema.json

tests/e2e_test.go

tests/test.tsr