feat(tls): Allow the underlying TLS implementation to be selected by feature flags.

[stevenj]

Content

The mithril-client library uses the reqwest crate. By default, reqwest uses OpenSSL for TLS support in HTTPS requests. This causes any consumer of the library to have a dynamic dependency on a system OpenSSL library.

reqwest has a number of options for fulfilling tls functionality. This patch just exposes that, using the same feature flags that reqwest uses.

Pre-submit checklist

• Branch
  • [x] Tests are provided (if possible)
  • [x] Crates versions are updated (if relevant)
  • [x] CHANGELOG file is updated (if relevant)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested
• Documentation
  • [x] Update README file (if relevant)
  • [ ] Update documentation website (if relevant)
  • [ ] Add dev blog post (if relevant)

Comments

This PR came about because our project requires static linking of dependent libraries, and this was not possible when using the mithril-client crate.

Issue(s)

Closes #1737

[jpraynaud]

@stevenj could you bump the patch version of the mithril-client crate before we can merge the PR?