Add HybridCache public API

[mgravell]

This is the final proposed API for HybridCache (#54647) as discussed #54647

The implementation is a placeholder (that just invokes the backend service every time, i.e. no cache), and exists purely to test (via the tests project) that:

• the primary service configuration API can be used through code
• the configuration options are compatible with ConfigurationBinder
• the primary GetOrCreateAsync APIs can be used through code (in particular with implicit generic typing)
• it all works on .NET Framework in addition to .NET "current"
• the serializer API can at least be implemented and configured successfully (not actually used anywhere)

The code incorporates the changes and feedback from #54647

The bits that we anticipate pushing to runtime are in a sub-folder, but are in their intended namespaces.

[amcasey]

I have some questions but it seems generally good to me. @halter73?

[BrennanConroy]

Mostly nits about readability and coding conventions we use

[Migaroez]

Saw a typo while reading the reviews
DisableUndelyingData => DisableUnderlyingData

I did not check all references just did a quick find on github

[mgravell]

@Migaroez check that you're looking at latest - see 7edddb1

[mgravell]

@halter73 have I addressed all of your concerns?

[mgravell]

@joegoldman2 thanks for all the great documentation tweaks

[mgravell]

All issue feedback has been addressed, and I've invited additional @halter73 - I'm interpreting existing tick plus silence as "fine"; merging; happy to make additional tweaks as required, but would rather hit next preview and then have a break, than not hit the next preview