Add support for TimeStreamWrite and TimeStreamQuery (#2707)

TODO:
- [x] docs
- [x] integration test (canary even?)
- [x] customize README for timestream
## Motivation and Context
- #613 
- https://github.com/awslabs/aws-sdk-rust/issues/114


## Description
This adds support for TSW and TSQ by adding endpoint discovery as a
customization. This is made much simpler by the fact that endpoint
discovery for these services **has no parameters** which means that
there is no complexity from caching the returned endpoint.

Customers call `.enable_endpoint_discovery()` on the client to create a
version of the client with endpoint discovery enabled. This returns a
new client and a Reloader from which customers must spawn the reload
task if they want endpoint discovery to rerun.

## Testing
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## Checklist
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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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