aip/4114: read and parse context_aware_metadata.json file

[coryan]

If the GOOGLE_API_USE_CLIENT_CERTIFICATE environment variable is set to true we need to find, read, and parse the context_aware_metadata.json file. This is (as the name implies) a JSON file, most likely introducing a new dependency for the gRPC-based libraries.

https://google.aip.dev/auth/4114

[coryan]

We should do this at some point

[coryan]

Strike 2, we still want to do this.

[coryan]

We do not have time to work on this, closing.