feat: WIP optimize url creation

[TimothyMakkison]

WIP; Uses the first call to determine the substitutions for the URL.

• This might be a "breaking change" as the url conversion is done later on and in bulk, in addition to calling UrlParameterFormatter.Format on the same value if it appears several times in the URL.
  • If this is an issue I can use a ValueListBuilder to preserve the order of operations and to cache values,
• ParameterFragment is my messy attempt to create a tagged union.
  • Instead of having a string Value property it could be an index and count representing the reevant area in the original url.
• Uses ValueStringBuilder I'll need to rebase this after feat: use ValueStringBuilder adding the query parameters #1719 is merged.
• This would ideally be done in the source generator.
• Could do with some benchmarks, especially for startup times.

[codecov]

Codecov Report

Attention: Patch coverage is 93.43066% with 9 lines in your changes missing coverage. Please review.

Project coverage is 84.59%. Comparing base (6ebeda5) to head (39e0db7).
Report is 43 commits behind head on main.

Files	Patch %	Lines
Refit/RestMethodInfo.cs	94.11%	3 Missing and 2 partials ⚠️
Refit/RequestBuilderImplementation.cs	92.30%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1730      +/-   ##
==========================================
- Coverage   87.73%   84.59%   -3.14%     
==========================================
  Files          33       35       +2     
  Lines        2348     2487     +139     
  Branches      294      327      +33     
==========================================
+ Hits         2060     2104      +44     
- Misses        208      299      +91     
- Partials       80       84       +4     

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