Simplify default config and add default async sleep (#3071)

While implementing identity caching, I noticed we don't have a default
async sleep impl wired up for generic clients, which was causing caching
to panic in many tests since it needs a sleep impl for timeouts. I
likely need to figure out what to do other than panic if there's no
sleep impl, but that's a problem for a different PR.

This PR adds a default sleep impl to generic clients, and also
simplifies how default config works. Previously, the generated config
`Builder::build` method set all the defaults with a series of "if not
set, then set default" statements. In this PR, defaults are registered
via default ordered runtime plugins.

Additionally, I cleaned up the standard retry strategy:
- The `TokenBucketPartition` didn't appear to be used at all, so I
deleted it.
- `StandardRetryStrategy` was taking retry config at construction, which
means it isn't possible to config override the retry config unless the
strategy is recreated. It now doesn't take any config at all during
construction.
- The adaptive retry client rate limiter was created at construction
based on retry config at that point in time. This means config overrides
wouldn't work with it, so it is also no longer set up at construction
time.
- Removed some unused runtime plugins.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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