* App Attest provider: attestation sequence (#761)

* App Attest draft WIP

* FIRAppAttestProvider initializers

* ./scripts/style.sh

* FIRAppAttestProvider implementation draft

* Basic FIRAppAttestProviderTests and fixes

* style

* testGetTokenWhenAppAttestIsNotSupported

* More FIRAppAttestProviderTests

* Cleanup

* Remove unused file

* Availability annotations on DCAppAttestService category.

* Guard FIRAppAttestProvider with #if TARGET_OS_IOS

* Formatting

* Fix SPM

* app_check.yaml: Add diagnostics SPM builds

* fix yaml

* Fix Firebase-Package scheme bad merge

* Fix typo

* FIRAppAttestProvider: hide default init

* FIRAppAttestKeyIDStorage: methods placeholders

* Comments

* Fix updated block definition

* Implement FIRAppAttestKeyIDStorage

* Add FIRAppAttestKeyIDStorageTests

* Review [Draft]

* Style

* Docs updates

* Docs updates 2

* Review [Draft] 2

* Improve tests

* Improve test readability

* Improve test readability 2

* Handshake adjustments (WIP)

* Introduce FIRAppAttestProviderState

* WIP: calculate attestation state

* WIP: calculate attestation state 2

* formatting

* Comments and moving code around

* Fix init in tests

* Fix state calculation flow

* Cleanup state calculation and fix tests.

* Cleanup and fixes.

* Comments

* formatting

* Fix import

* Typo fixes and additional comments

* FIRAppAttestInitialHandshakeResponse API

* Cleanup state calculation using FBLPromiseAwait

* Cleanup

* style

* Update comments

* FIRAppAttestArtifactStorage implementation and tests

* Fix init

* API docs

* Clean up storage in tests

* Comments

* Disable Keychain dependent tests for SPM

* Initial implementation

* Parse response body for challenge and stub test cases

* Review [Draft]

* Avoid encoding challenge again

* Add tests

* Revert "Avoid encoding challenge again" and add TODO

This reverts commit 69eb00d.

* Document tests; Add test

* Tests: Add URL validation check

* Review

* App Attest provider API integration WIP

* update tests

* Draft attestation response parsing

* Attestation request draft

* style

* AppAttest Attestation API tests draft

* Error cases tests

* style

* Cleanup and API docs

* Merge fix

* Fix OCMock imports

* Fix nullability modifier

* Formatting

* comments

* calculatre sha256 of random challenge for attestation

* Test app adjustments

* cleanup

* use trailing closures in the test app

…main

* Implement assertion exchange

* Tweak existing tests

* Add tests

* Rename JSON to better match gRPC  message

* Add HTTPBody helper

* Review

* Review 2

* Review 3

…main

…e-ios-sdk into appcheck-appattest-main

* App Attest assertion workflow draft

* send request

* assertion flow tests

* style

…main

* Update artifact storage API and tests

* Artifact storage implementation update

* Save artifact for a key ID

* Style

* typos

* App Attest multiple get token method invocation tests

* Ensure a single App Attest handshake sequence at the time

* FIRAppCheckTests: get token request merging tests

* FIRAppCheck: Ensure a single get token operation at the time

* formatting

* Test new request after merged requests

* Release finished operation promise

* Style

* Typos

* typo

* Request merging tests for error cases

* formatting

…main

* Remove/update outdated TODOs

* [WIP] Attestation rejection handling draft

* style

* retry tests draft

* reset key ID before retry

* Reset attestation

* test error and fixes

* style

* More details in the name

* Some debug logging

* style

* Use specific codes for log messages

* style

…s-sdk into mm/appcheck-refresh