feat: Make encoding_rs an optional feature

[Marwes]

Keeping it as a default feature so that the default user experience isn't to be confused about the lack of text. It is possible that text (but not text_with_charset) exists without the encoding_rs feature by only trying to decode the response as utf-8 in that case. However that would mean that the behaviour changes if the feature is later added since the method would start to respect Content-Type then.

Closes #1785

[seanmonstar]

If you base it off the 0.12-dev branch, it should clear up the conflicts. (Or it might need a rebase or a new branch and cherry-pick the commit, unsure). That's where I'm staging all the breaking changes.

[Marwes]

Getting some test failures because text is now missing with --no-default-features, unsure how you want this to be fixed, changing the tests to use bytes(), disable them for this case or to try and enable a limited form of text() without encoding_rs.

Feel free to close this PR and take or remake the changes if that is easier.

[seanmonstar]

I think I'm leaning towards text just using from_utf8 without default features, and documentation that says it can decode more if you enable the charset feature. What do you think?

[seanmonstar]

Wonderful! Thanks!