Update ConfigurableOptions to read from ConfigProperties

[psx95]

Changes

• Updates the GCP auth config to read from ConfigProperties instead of directly reading environment variables or system properties.
• Adds log statements informing users of possible misconfiguration of the GOOGLE_OTEL_AUTH_TARGET_SIGNALS.

Existing Issue(s):

Fixes #1903

Testing:

• Existing unit tests pass indicating that reading from ConfigProperties works as expected.
• Manually tested this extension with sample application to ensure successful exports.

Documentation:

• Internal implementation change only, no documentation updates needed.

Outstanding items:

• Manual testing post merge of Add metrics support to auth extension #1891 .

[jsuereth]

Overall looks good. Capitalization handling - here be dragons. Can you update your goals here for supported environment variable names so it's easier to evaluate if this will break in international contexts?