Enforce the same minimum TLS version (1.2) for both TLS backends (#2312)
* Enforce the same minimum TLS version (1.2) for both TLS backends
* Add CHANGELOG entry
* Add documentation for both `https` and `native_tls`.
* Remove unnecessary mut
Provide a way to retrieve fallback credentials if a call to `provide_credentials` is interrupted. An interrupt can occur when a timeout future is raced against a future for `provide_credentials`, and the former wins the race. A new method, `fallback_on_interrupt` on the `ProvideCredentials` trait, can be used in that case. The following code snippet from `LazyCredentialsCache::provide_cached_credentials` has been updated like so: