Support rustls with feature flag via reqwest

[skeggse]

What does this PR do?

Adds a feature flag to control the TLS implementation used internally by the client to fix #157.

Additional Notes

Review checklist

Please check relevant items below:

• This PR includes all newly recorded cassettes for any modified tests. (N/A)
• This PR does not rely on API client schema changes.
  • The CI should be fully passing.
• Or, this PR relies on API schema changes and this is a Draft PR to include tests for that new functionality.
  • Note: CI shouldn't be run on this Draft PR, as its expected to fail without the corresponding schema changes.

[nkzou]

Hi there, thanks for your contribution - we're planning to cut a new release next week, so it should be updated on crates.io by then.

[skeggse]

Thank you!