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]

	Fails
🚫	Please consider adding a changelog entry for the next release.

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

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))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description or adding a skip-changelog label.

Generated by 🚫 dangerJS against d9fc764

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.
✅ 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