feat: add async cert validation support

[CarolYeh910]

Release Summary:

Adds async support to s2n_cert_validation_callback

Description of changes:

The async validation function should throw an S2N_ERR_ASYNC_BLOCKED error when neither s2n_cert_validation_accept() or s2n_cert_validation_reject() are called by the validation callback. After the application finishes processing and updates the callback status (i.e. the cert_validation_info struct) using either API, the application can retry the handshake and complete the validation. To implement this feature, we should consider how to handle the re-entry case. Below are the changes I made:

1. Moved the cert_validation_info struct into s2n_x509_validator to extend its lifetime, so we can update or check the status after the callback is finished.
2. Created a bool variable cert_validation_cb_invoked in s2n_x509_validator to identify a re-entry as well as a helper function s2n_x509_validator_handle_cert_validation_callback_result() to check the callback status (e.g. if cert_validation_info.finished is false, bail with S2N_ERR_ASYNC_BLOCKED).
3. Pulled the major validation process into a new function and updated the control flow with the bool variable in s2n_x509_validator_validate_cert_chain(). On the re-entry case, we only check callback status and do not repeat the validation process.
4. Retrying the handshake will trigger s2n_handle_retry_state(), which invokes s2n_server_cert_recv() or s2n_client_cert_recv() without updating the stuffer read cursor. It may cause stuffer out of data while retrying these two functions. One solution is to copy the stuffer from conn->handshake.io and restore it after the validation is completed.
5. Added async test cases (in both TLS 1.3 and 1.2 versions) and implemented self-talk unit tests for the async use case.

Testing:

Confirmed the self-talk unit tests passed both on the client side and on the server side.

$ cmake --build build/ --target test -- ARGS="-L unit -R s2n_cert_validation_callback_test --output-on-failure"

Running tests...
Test project /home/ubuntu/s2n-tls/build
    Start 18: s2n_cert_validation_callback_test
1/1 Test #18: s2n_cert_validation_callback_test ...   Passed    0.73 sec

100% tests passed, 0 tests failed out of 1

Label Time Summary:
unit    =   0.73 sec*proc (1 test)

Total Test time (real) =   0.74 sec

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.