Make normalize_path configurable to allow for trailing slashes

[alper]

I think paths should be normalised to end with a slash.

See how (and why) Django does this:

• https://stackoverflow.com/a/42213107/102315
• https://docs.djangoproject.com/en/dev/ref/settings/#std-setting-APPEND_SLASH

Since tower-http/src/normalize_path.rs has already gone in the other direction, I guess the best way forward would be to make it configurable so that those who prefer trailing slashes can get them?

I'll happily make the change.

[jplatte]

The existing constructor is named trim_trailing_slash (rather than new) to allow for other constructors / normalization styles to be added down the line. So feel free to send a PR. I think I had a short discussion with somebody around what to name the "add trailing slash if missing" style before, but I don't know where that was. If it's not in another issue or GH discussion, it's probably going to be impossible to find. Settling the exact name can happen on an otherwise finished PR, IMHO.