Proxy a provider-native model request

The provider's native API path verbatim, without a leading slash.

Required for gateway-token clients; optional for issuer-less API-key clients.

Required on every request sent with the gateway token of an App Attest application whose authentication.app_attest.verify_on is every_request; ignored, and never forwarded, where verify_on is token_exchange and for API-key clients. Format v1.<timestamp>.<nonce>.<assertion>: decimal unix seconds from the device clock (signed, but not compared with the gateway's clock), 16 random bytes as base64url without padding, and the CBOR assertion from DCAppAttestService.generateAssertion as base64url without padding. The assertion's client data is the SHA-256 of these lines joined by \n with no trailing newline: appaigateway-request-v1, the app id, the upper-case HTTP method, the URL path and query exactly as sent, base64url(sha256(body bytes)) (an absent body hashes as empty), base64url(sha256(the bearer token)), the timestamp, and the nonce. Missing answers 401 assertion_required; refused answers 403 attest_failed. The AppAIGateway Swift package 1.4 or later computes it.

Optional configured end-user identity for issuer-less API-key applications.