google/google: update documentation for workload identity federation
google/google: update documentation for workload identity federation

Including information on executable-sourced credentials

Change-Id: I39bcf20ffd1f5a9026d3d18e127411c03021977d
GitHub-Last-Rev: d61f2e71d26d9111f0fecd481ee1615ac60ad49b
GitHub-Pull-Request: golang/oauth2#592
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/436836
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Leo Siracusa <768d6c74aea8dac579c46c3210e3242f16aae951@google.com>
Run-TryBot: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>
Reviewed-by: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>
Auto-Submit: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>

golang/oauth2