impl(oauth2): validate some URLs in AWS external account

[coryan]

The spec (aip.dev/auth/4117) requires validation for some URL fields in the AWS external account configuration.

Part of the work for #5915

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 6715b77f2f07b3f56987fb1e791252218d33c64a

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 93.87% // Head: 93.87% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (1941a11) compared to base (b80a5f2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10448   +/-   ##
=======================================
  Coverage   93.87%   93.87%           
=======================================
  Files        1612     1612           
  Lines      146520   146572   +52     
=======================================
+ Hits       137540   137593   +53     
+ Misses       8980     8979    -1     

Impacted Files	Coverage Δ
...loud/internal/external_account_token_source_aws.cc	93.38% <100.00%> (+0.93%)	⬆️
...internal/external_account_token_source_aws_test.cc	99.72% <100.00%> (+0.03%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 1941a116855de095ba9d11d259501cbb664c6251

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.