Support dynamic HTTP headers

[martincostello]

• Add support for dynamically matching and producing HTTP headers.
• Add missing null check.
• Use GITHUB_REF_NAME instead of manipulating GITHUB_REF.

Resolves #473.

[codecov-commenter]

Codecov Report

Merging #475 (9c44ab0) into main (fca25e8) will decrease coverage by 0.37%. The diff coverage is 94.82%.

@@             Coverage Diff             @@
##              main     #475      +/-   ##
===========================================
- Coverage   100.00%   99.62%   -0.38%     
===========================================
  Files           16       16              
  Lines          753      790      +37     
===========================================
+ Hits           753      787      +34     
- Misses           0        3       +3     

Flag	Coverage Δ
linux	99.62% <94.82%> (-0.38%)	:arrow_down:
macos	99.62% <94.82%> (-0.38%)	:arrow_down:
windows	?

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Impacted Files	Coverage Δ
...ientInterception/HttpRequestInterceptionBuilder.cs	98.88% <94.82%> (-1.12%)	:arrow_down:

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