feat: add support for counting tokens

[StevanFreeborn]

closes #19
closes #25

• fix: remove model id validation
• feat: add support for count tokens endpoint
• refactor: put public method above private methods
• tests: add integration tests to cover both 😁 and 🥲 paths when counting message tokens
• chore: remove need for beta header when using prompt caching or PDF support
• docs: add count message tokens example to README.md

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.57%. Comparing base (9c0a2e3) to head (de326ab).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/AnthropicClient/AnthropicApiClient.cs	85.71%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   98.71%   98.57%   -0.15%     
==========================================
  Files          66       67       +1     
  Lines        1247     1264      +17     
  Branches      101      103       +2     
==========================================
+ Hits         1231     1246      +15     
  Misses          5        5              
- Partials       11       13       +2     

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