Refactor Vendor Plugin

[daymxn]

Per b/391351857,

This refactors the VendorPlugin and VendorTask to be more compliant with the gradle cache and configuration avoidance. It also adds proper task logging and documentation to both the task and the plugin.

This PR also fixes the following:

• b/391352175 -> Rename plugin Util files to Extensions

[github-actions]

Release note changes

No release note changes were detected. If you made changes that should be
present in the next release, ensure you've added an entry in the appropriate
CHANGELOG.md file(s).

[github-actions]

Vertex AI Mock Responses Check ⚠️

A newer major version of the mock responses for Vertex AI unit tests is available. update_responses.sh should be updated to clone the latest version of the responses: v5.3

[github-actions]

plugins test results

37 tests   37 ✅  2m 18s ⏱️
 8 suites   0 💤
 8 files     0 ❌

Results for commit 9be5a54.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results

 1 034 files  ±0   1 034 suites  ±0   34m 37s ⏱️ +25s
 5 888 tests ±0   5 866 ✅ ±0  22 💤 ±0  0 ❌ ±0 
11 839 runs  ±0  11 795 ✅ ±0  44 💤 ±0  0 ❌ ±0 

Results for commit 9be5a54. ± Comparison against base commit 377edcc.

♻️ This comment has been updated with latest results.

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (377edcc) and merge commit (89e8435).

Test Logs

• Base (377edcc): https://github.com/firebase/firebase-android-sdk/actions/runs/12932999795
• Merge (89e8435): https://github.com/firebase/firebase-android-sdk/actions/runs/12954505442

1. https://storage.googleapis.com/firebase-sdk-metric-reports/l88vjyjzVz.html

[google-oss-bot]

Coverage Report ¹

Affected Products

• firebase-database

  Overall coverage changed from 50.21% (377edcc) to 50.16% (89e8435) by -0.04%.

  Filename	Base (377edcc)	Merge (89e8435)	Diff
  ChildChangeAccumulator.java	96.67%	83.33%	-13.33%

• firebase-storage

  Overall coverage changed from 83.92% (377edcc) to 83.96% (89e8435) by +0.04%.

  Filename	Base (377edcc)	Merge (89e8435)	Diff
  StreamDownloadTask.java	88.41%	88.89%	+0.48%

Test Logs

• Base (377edcc): https://github.com/firebase/firebase-android-sdk/actions/runs/12932999795
• Merge (89e8435): https://github.com/firebase/firebase-android-sdk/actions/runs/12954505442

1. https://storage.googleapis.com/firebase-sdk-metric-reports/gDHGUlZ7Xo.html