Update runtime plugin trait (#2754)

## Motivation and Context
Update the RuntimePlugin trait based on discussion:
1. Methods are infallible
2. Split out `Config` and `Interceptors`
3. `ConfigBag` now has an explicit field `interceptor_state`
4. Refactor `ConfigBagAccessors` so that we can build the core around
the trait and keep the trait together with a `where Self` trick

## Description
- Update the `RuntimePlugin` trait
- Deal with resulting implications
## Testing
- [x] CI


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Co-authored-by: John DiSanti <jdisanti@amazon.com>