Move Azure cache implementation to plugin

[Copilot]

• Create change file for the PR with all modified packages and appropriate semver types
• Address PR review comments:
  • 1. Share CacheStorage class from backfill-cache instead of duplicating in azure-blob-cache-storage
  • 2. Share hashFile from backfill-cache instead of duplicating in azure-blob-cache-storage
  • 3. Delete .npmignore from azure-blob-cache-storage (not needed with package.json files)
  • 4. Resolve plugin path relative to cwd in getCacheStorageProvider.ts
  • 5. Remove the old CustomStorageConfig from cacheConfig.ts and all references
  • 6. Use "email not defined" in change file instead of a random email
• Address follow-up review comments:
  • 7. Restore original comments in CredentialCache.ts
  • 8. Remove cacheStorageConfig override in CacheOptions.ts
  • 9. Move plugin implementation from index.ts to separate file
  • 10. Fix mock plugin string in api.test.ts
• Remove disallowedChangeTypes from beachball.config.js to allow major version bumps

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Move Azure cache implementation to plugin</issue_title>
<issue_description>For the many users of lage (and indirectly backfill) who don't use remote caching, it would be nice if the Azure cache implementation could be moved to a plugin instead of being included by default (due to the significant dependency+parsing bulk). Making this approach extensible would also open the opportunity for other remote cache backends.

Probably something like this:

interface CustomCacheStoragePlugin<TOptions> {
  name: string;
  getProvider: (logger: Logger, cwd: string, options: TOptions) => ICacheStorage;
}

interface CustomCacheStorageConfig<TOptions = unknown> {
  provider: "custom";
  // Can be either a path or a package name.
  // If a package name, it's resolved from the location of the config file.
  // (Maybe come up with a better prop name...?)
  plugin: string;
  options: TOptions;
}

// REMOVED: AzureBlobCacheStorageConfig (now a plugin)
type CacheStorageConfig =
  | { provider: "local" }
  | { provider: "local-skip" }
  | NpmCacheStorageConfig
  | CustomCacheStorageConfig;

// lage.config.js
const lageConfig = {
  cacheOptions: {
    cacheStorageConfig: {
      provider: "custom",
      plugin: "@lage-run/azure-blob-cache-storage",
      options: { /*...*/ }
    }
  }
}
```</issue_description>

<agent_instructions>1. Update the types in packages/backfill-config/src/cacheConfig.ts as described in the issue.
2. Move all the Azure-related types, code, and dependencies from `packages/backfill-cache` to a new package `packages/azure-blob-cache-storage` (name: `@lage-run/azure-blob-cache-storage`). `backfill` and `lage` should not depend on this new package.
3. Update documentation (`docsite` plus any mentions in readmes) to describe the new approach</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Move Azure cache implementation to plugin #1037

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