Cleanup internals

[martintmk]

Details on the issue fix or feature implementation

Cleaning and unifying internal implementation following the recent changes to pipeline composition.

• Introducing an internal PipelineComponent that represents an executable unit of a pipeline. It can be created from:
  • ResilienceStrategy (generic/non-generic)
  • ResiliencePipeline (generic/non-generic)
• ResiliencePipeline (generic/non-generic) is now sealed and with internal constructor
• Tidying and unifying some internals
• Dropped NullResilienceStrategy.Instance in favor of ResilienceStrategy.Null (similar to Stream.Null)

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

[codecov]

Codecov Report

Merging #1492 (166f635) into main (1d16fd7) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1492      +/-   ##
==========================================
- Coverage   83.63%   83.61%   -0.02%     
==========================================
  Files         272      267       -5     
  Lines        6390     6385       -5     
  Branches     1010     1007       -3     
==========================================
- Hits         5344     5339       -5     
  Misses        837      837              
  Partials      209      209              

Flag	Coverage Δ
linux	83.61% <100.00%> (-0.02%)	⬇️
macos	83.61% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/Polly.Core/ResiliencePipeline.Async.cs	100.00% <ø> (ø)
src/Polly.Core/ResiliencePipeline.AsyncT.cs	100.00% <ø> (ø)
src/Polly.Core/ResiliencePipeline.Sync.cs	100.00% <ø> (ø)
src/Polly.Core/ResiliencePipeline.SyncT.cs	100.00% <ø> (ø)
...ore/Registry/ResiliencePipelineRegistry.TResult.cs	100.00% <100.00%> (ø)
.../Polly.Core/Registry/ResiliencePipelineRegistry.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResiliencePipeline.cs	100.00% <100.00%> (ø)
...rc/Polly.Core/ResiliencePipelineBuilder.TResult.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResiliencePipelineBuilder.cs	100.00% <100.00%> (ø)
src/Polly.Core/ResiliencePipelineBuilderBase.cs	100.00% <100.00%> (ø)
... and 10 more