feat: enable non-'static where possible

[Daniel-Aaron-Bloom]

Motivation

Currently, many tower service wrappers and layers are limited to working with 'static types, which limits its usefulness when working with non-statically borrowed things to and from scoped tasks (e.g. async-scoped).

Solution

Remove the 'static restrictions where possible, mostly by introducing lifetime parameters.

This change excludes Buffer and BufferLayer which are more difficult to make non-'static.

This PR replaces #732

[jplatte]

As a co-maintainer, I am very hesitant about merging breaking changes. I want to leave this up to people who have been involved with the project more (unfortunately that probably means there won't be a decision here for a long time).

[seanmonstar]

That's what I kind of thought when looking at this too, it's a breaking change. We could consider it for the next major release, but I don't know when that is.

[ripytide]

It's good progress at least though if you maintainers are happy with the change given the requirement that it is merged as part of the next major breaking release and not merged until then.

[Daniel-Aaron-Bloom]

Are there any labels that could be applied to this PR to ensure it doesn't get forgotten in vNext?