add endpoint for public key and return signed tree head with loginfo (#97)

cmd/cli/app/log_info.go

cmd/cli/app/root.go

openapi.yaml

pkg/api/error.go

pkg/api/tlog.go

pkg/generated/client/tlog/get_public_key_parameters.go

pkg/generated/client/tlog/get_public_key_responses.go

pkg/generated/client/tlog/tlog_client.go

pkg/generated/models/log_info.go

pkg/generated/restapi/configure_rekor_server.go

pkg/generated/restapi/doc.go

pkg/generated/restapi/embedded_spec.go

pkg/generated/restapi/operations/rekor_server_api.go

pkg/generated/restapi/operations/tlog/get_public_key.go

...ated/restapi/operations/tlog/get_public_key_parameters.go

...rated/restapi/operations/tlog/get_public_key_responses.go

...ated/restapi/operations/tlog/get_public_key_urlbuilder.go