Allow adding generic strategies to generic builder

[martintmk]

Details on the issue fix or feature implementation

Noticed that this API is missing while testing the latest alpha.

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

[martincostello]

Good catch.

[codecov]

Codecov Report

Merging #1386 (04019d3) into main (22df640) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1386   +/-   ##
=======================================
  Coverage   83.68%   83.68%           
=======================================
  Files         269      269           
  Lines        6429     6429           
  Branches     1004     1003    -1     
=======================================
  Hits         5380     5380           
  Misses        840      840           
  Partials      209      209           

Flag	Coverage Δ
linux	83.68% <100.00%> (ø)
macos	83.68% <100.00%> (ø)
windows	?

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Impacted Files	Coverage Δ
.../Polly.Core/ResilienceStrategyBuilderExtensions.cs	100.00% <100.00%> (ø)

... and 8 files with indirect coverage changes