Add HttpsConnector::https_only for compatibility with hyper_tls

[jjant]

Description

It's convenient to be able to configure this flag after building the HttpsConnector, and hyper_tls has this functionality so this is nice for compatibility.

See smithy-lang/smithy-rs#2316 (comment) for motivation.

[djc]

So I don't mind the use case of enabling force_https after construction, but this API can also be used to do the opposite, which seems potentially trickier? Like, any caller with a &mut HttpsConnector can now stop enforcing the use of HTTPS, which might be the opposite of the caller intentions when passing in a fully-formed HttpsConnector.

If you change this to making it unidirectional (maybe enforce_https()?), I don't see any issues.

[jjant]

Thanks for the suggestion, @djc, I've made the change in 6f83fc5.

[djc]

Thanks!