feat: add traces_sampler

[JoshuaMoelans]

Fixes #864

• Add to options
• Add sentry_traces_sampler_function typedef (like the on_crash and before_send ones
• Add samplingContext
• Update docs (tracing and sampling)

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 7344f8a

[codecov]

Codecov Report

Attention: Patch coverage is 15.38462% with 22 lines in your changes missing coverage. Please review.

Project coverage is 82.42%. Comparing base (b24b30c) to head (7344f8a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1108      +/-   ##
==========================================
- Coverage   82.58%   82.42%   -0.16%     
==========================================
  Files          53       54       +1     
  Lines        7896     7921      +25     
  Branches     1234     1237       +3     
==========================================
+ Hits         6521     6529       +8     
- Misses       1265     1281      +16     
- Partials      110      111       +1     

[JoshuaMoelans]

Do we want to provide something similar to the cocoa SDK, or is it up to the users to define what type and how they'll use it in the tracesSampler callback?

e.g.

typedef struct {
    char *key;
    void *value;
} custom_sampling_context_entry_t;

typedef struct {
    custom_sampling_context_entry_t *entries;
    size_t entry_count;
} custom_sampling_context_t;