V1 custom code

[bpapillon]

This pull request implements Schematic V1 functionality for Schematic's C# library; V1 adds the following custom functionality:

• Configurable in-memory caching for flag check evaluations (users should get an in-memory cache for flag check results by default, which they can disable, configure its TTL or max size, or even bring their own cache implementation)
• Event buffering (i.e., track and identify events can be submitted via a non-blocking top-level function, and will be sent in batches behind the scenes)
• Offline mode (to support local development environments without a Schematic API key, CI testing environments, etc)

[bpapillon]

@bpapillon TODO:

• add github action for tests
• confirm dependency change with Fern

@nasimsaleh TODO:

• small change (inline comment) for cache key
• update README to reflect interface changes (eg changes to cache, TTL type)