feat: Make transport channel capacity configurable

[lcian]

Description

In high throughput scenarios it's likely to saturate the capacity of the transport channel, in particular when capturing many transactions, as each of them will form a single envelope for what might be just a single span. Right now, the capacity is hardcoded to 30. This introduces a new option that allows users to override the default capacity.

[github-actions[bot]]

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:no_entry_sign:	Please consider adding a changelog entry for the next release.

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Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

### Features

- Make transport channel capacity configurable ([#923](https://github.com/getsentry/sentry-rust/pull/923))

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Generated by :no_entry_sign: dangerJS against d9fc76403fd6366511fb880bb25c0be5c5aa157d

[codecov[bot]]

Codecov Report

:x: Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 73.75%. Comparing base (ca23268) to head (d9fc764).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
- Coverage   73.79%   73.75%   -0.04%     
==========================================
  Files          64       64              
  Lines        7544     7548       +4     
==========================================
  Hits         5567     5567              
- Misses       1977     1981       +4