feat: calculate path substitutions in `RestMethodInfo`

[TimothyMakkison]

Parse the url target inside RestMethodInfo, removing the use of regex on function calls.

Re attempt at #1730. Turns out the additional allocations were caused by line 829 creating a closure. Looking at the lowered code C# always created the linq closure, even when round tripping was never used 🤔

In future I could optimise round trip to not use string.Split, either manually splitting or using Span.Split

Benchmarks (on battery power)

Method	Mean	Error	StdDev	Median	Gen0	Allocated
ConstantRouteAsync	9.332 us	0.6444 us	1.8999 us	9.479 us	0.2899	2.71 KB
DynamicRouteAsync	5.284 us	0.2704 us	0.7449 us	4.957 us	0.3052	3 KB
ComplexDynamicRouteAsync	5.091 us	0.4201 us	1.2387 us	4.433 us	0.3662	3.41 KB
ObjectRequestAsync	5.942 us	0.1163 us	0.2504 us	5.834 us	0.4730	4.41 KB
ComplexRequestAsync	18.005 us	0.4408 us	1.2359 us	17.392 us	1.1292	10.44 KB

Old benchmarks

Method	Mean	Error	StdDev	Gen0	Allocated
ConstantRouteAsync	2.072 us	0.0110 us	0.0097 us	0.2937	2.71 KB
DynamicRouteAsync	2.783 us	0.0259 us	0.0202 us	0.3319	3.07 KB
ComplexDynamicRouteAsync	4.060 us	0.0226 us	0.0200 us	0.4044	3.78 KB
ObjectRequestAsync	4.954 us	0.0295 us	0.0262 us	0.4807	4.48 KB
ComplexRequestAsync	14.455 us	0.0686 us	0.0642 us	1.1597	10.67 KB

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.52055% with 8 lines in your changes missing coverage. Please review.

Project coverage is 84.89%. Comparing base (6ebeda5) to head (288ad31). Report is 156 commits behind head on main.

Files with missing lines	Patch %	Lines
Refit/RestMethodInfo.cs	94.11%	3 Missing and 2 partials :warning:
Refit/RequestBuilderImplementation.cs	95.08%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1897 +/- ## ========================================== - Coverage 87.73% 84.89% -2.85% ========================================== Files 33 36 +3 Lines 2348 2502 +154 Branches 294 361 +67 ========================================== + Hits 2060 2124 +64 - Misses 208 299 +91 + Partials 80 79 -1 ```

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