In-memory caching for event readers

[Grepsy]

Initial draft for an in-memory caching layer that wraps an IEventFeedReader based on https://github.com/dburriss/event-feed/issues/2.

Some considerations:

• Complete pages now still expire within an absolute expiration time. It would be nicer if we could allocate a fixed size memory budget to keep pages in memory as long as we're within budget but this seems tricky as calculating object memory usage is required to set the Size property on MemoryCacheEntryOptions. A possible solution would be to estimate this based on the payload size.
• Usage-metrics are kept out of scope but this impl. should not prevent that.
• No worker-based cache refreshing is done, see https://github.com/dburriss/event-feed/issues/2#issuecomment-1092052140.
• F# might not be idiomatic, feedback appreciated!

Further work:

• Tests + docs
• ASP.NET Core helpers to easily add this.
• Make expiration options configurable.
• Support new event notification and update the in-memory cache.
• Possibly abstract the IMemoryCache to support distributed caching scenarios (seems like this can be done when actually required).
• Discuss back-channel notifications (needs more information).