feat: expose transport utilities

[thomaseizinger]

Description

Exposes RateLimiter, RateLimiterCategory, StdTransportThread and TokioTransportThread from the transports module.

Issues

Resolves: #941

Reminders

• Add GH Issue ID & Linear ID (if applicable)
• Add an entry to CHANGELOG.md, following the format of existing entries
• The PR title should use Conventional Commits style (feat:, fix:, ref:, meta:, etc.)
• Useful links for external contributors: Sentry SDK development docs, Discord community