[gcp-auth-extension]: Try resolving GCP_PROJECT with ServiceOptions if not provided

[gustavovnicius]

Description:

Enhancement - Try resolving GOOGLE_CLOUD_PROJECT value using google-cloud-sdk.

Having a mandatory GOOCLE_CLOUD_PROJECT configuration is unnecessary, given the extension already depends on ADC context existing. It can try to infer the project from it.
Still mandatory to have a project id, but this gives more flexibility in the usage of the extension.

Existing Issue(s):
#2102

Testing:

Refactored the project id resolver logic into a separate method, including the more comprehensive ServiceOptions resolver. Tested the method for both the current strategy and falling back to ServiceOptions, also added a missing test for not being able to find the option and throwing an exception.

Documentation:

GOOGLE_CLOUD_PROJECT configuration was moved into the optional config section, with a note that even though it's optional, the extension needs it either provided or being able to infer it.

Outstanding items:

N/A

[linux-foundation-easycla]

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[otelbot-java-contrib]

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[otelbot-java-contrib]

❌ The result from spotlessApply could not be committed to the PR branch, see logs: https://github.com/open-telemetry/opentelemetry-java-contrib/actions/runs/16990824329.