Introduce IResilienceStrategy and core primitives for V8

[martintmk]

The issue or feature being addressed

In this PR I am transferring some core primitives from v8-poc branch. This includes:

• Adding documentation.
• Adding code-guards.
• Polishing and improving the API.

Details on the issue fix or feature implementation

To keep thing simple I am omitting the following:

• Pooling of ResilienceContext instances.
• Additional Execute extensions as there will be many of these (to be done in follow-up).

I'll include the tests once we agree on API shape.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[geeknoid]

Note that the default object pool in .NET has terrible scaling characteristics in scenarios where many pooled objects exist and are accessed in a concurrent fashion. The default pool was originally designed for very short-term object rentals and it just doesn't work well for long-term rentals.

I fixed this for .NET 8. In the meantime, you may wish to grab the .NET 8 implementation here and use it instead of the default .NET object pool.

https://github.com/dotnet/aspnetcore/blob/main/src/ObjectPool/src/DefaultObjectPool.cs

[martintmk]

My thinking is just to copy that code here since the revisited version is pretty simple. And we will avoid extra dependency.