Add open-source 'basic' cache provider and revamp licensing documentation (#930)

## Summary

- **New `basic` cache provider**: Adds an open-source (MIT-licensed)
caching implementation built on `@actions/cache` as an alternative to
the proprietary Enhanced Caching. Users can opt in with `cache-provider:
basic` on both `setup-gradle` and `dependency-submission` actions.
- **Revamped licensing & distribution docs**: Replaces the verbose
licensing notice block (previously shown in README, docs, and logs) with
a friendlier callout and a new dedicated
[DISTRIBUTION.md](./DISTRIBUTION.md) covering component licensing, usage
tiers, data privacy ("Safe Harbor"), and opt-out instructions.
- **Improved messaging**: Enhanced Caching and Basic Caching each
display concise, informative log messages and job summary notes instead
of the previous wall-of-text license warning.
- **New integration tests**: Adds `integ-test-basic-cache-provider.yml`
workflow that seeds and verifies the basic cache provider across
platforms, plus unit tests for `BasicCacheService` and `getCacheService`
selection logic.
- **CI workflow reorganization**: Dependency-submission integration
tests extracted into their own reusable suite
(`suite-integ-test-dependency-submission.yml`); sample project tests
moved into the caching suite.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/suite-integ-test-caching.yml

@@ -45,3 +45,21 @@ jobs:
     uses: ./.github/workflows/integ-test-restore-java-toolchain.yml
     with:
       skip-dist: ${{ inputs.skip-dist }}
+
+  sample-kotlin-dsl:
+    uses: ./.github/workflows/integ-test-sample-kotlin-dsl.yml
+    with:
+      runner-os: '${{ inputs.runner-os }}'
+      skip-dist: ${{ inputs.skip-dist }}
+
+  sample-gradle-plugin:
+    uses: ./.github/workflows/integ-test-sample-gradle-plugin.yml
+    with:
+      runner-os: '${{ inputs.runner-os }}'
+      skip-dist: ${{ inputs.skip-dist }}
+
+  basic-cache-provider:
+    uses: ./.github/workflows/integ-test-basic-cache-provider.yml
+    with:
+      runner-os: '${{ inputs.runner-os }}'
+      skip-dist: ${{ inputs.skip-dist }}