[Angular CSDK] ISR-like loader cache implementation

[art-alexeyenko]

Introduces a caching layer for loaders in Angular CSDK. It's designed to provide ISR-like behavior: cache entries for loaders are created and reused until becoming stale. When stale loader cache is accessed, it is revalidated (stale-while-revalidate, similar to next).
This is not a full fledged ISR implementation, however, and it's goal is to minimize Edge calls. Similarly, this does not introduce SSG support (it will be it's own can of worms later, but we have building blocks).

• adds ISR-like cache implementation
• cache contains loaders data (page, dictionary)
• support for OSR tag-based revalidation (following conventions from recent nextjs implementation)
• Temp Cache admin demo page for testing
• revalidation follows stale-while-revalidate principles
  • this includes changes to base loader resolver logic
  • two distinct loader flows: browser, server
  • cache only exists on server (we also ensure loaders themselves are only executed on server)
  • when cache entry ttl is reached or revalidate request received, cache entry is stale instead of removed (performance ++)
• tags support
  • there are two main cache implementations - default in-memory (similar to rjx-angular/isr) and unstorage
  • default one has a map of cache entries (with cache keys) and a collection of tag -> [cache keys] entries. This is how we find cache key by tag.
  • unstorage accepts a driver during implementation (memory, fs, redis etc) and uses it to invoke a storage. unstorage storage stores tags, cache keys and cache values in a similar way.
• cache is invoked in app, src/server.ts and cache object pointer is passed down into Angular SSR to ensure consistency.

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Latest commit: f1b5f30

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[sc-nikolaoslazaridis]

Good job!