Parameterize `Plugin` by service rather than protocol (#2772)

## Motivation and Context

Closes https://github.com/awslabs/smithy-rs/issues/1839

Currently, `Plugin` is parameterized by protocol and operation. To
improve symmetry, extensibility and uniformity we switch this to be
parameterized by service instead. The protocol can still be recovered
via the `type Protocol` associated type on `ServiceShape`.

## Description

- Add `ServiceShape` trait, encoding the properties of a Smithy service.
- Change `Plugin<Protocol, Operation, S>` to `Plugin<Service, Operation,
S>`.
- Add `FilterByOperation` and `filter_by_operation` `Plugin`s.