Allow server decorators to inject methods on config (#3111)

PR #3095 added a code-generated `${serviceName}Config` object on which
users can register layers and plugins. For example:

```rust
let config = PokemonServiceConfig::builder()
    .layer(layers)
    .http_plugin(authn_plugin)
    .model_plugin(authz_plugin)
    .build();
```

This PR makes it so that server decorators can inject methods on this
config builder object. These methods can apply arbitrary layers, HTTP
plugins, and/or model plugins. Moreover, the decorator can configure
whether invoking such method is required or not.

For example, a decorator can inject an `aws_auth` method that configures
some plugins using its input arguments. Missing invocation of this
method
will result in the config failing to build:

```rust
let _: SimpleServiceConfig<
    // No layers have been applied.
    tower::layer::util::Identity,
    // One HTTP plugin has been applied.
    PluginStack<IdentityPlugin, IdentityPlugin>,
    // One model plugin has been applied.
    PluginStack<IdentityPlugin, IdentityPlugin>,
> = SimpleServiceConfig::builder()
    // This method has been injected in the config builder!
    .aws_auth("a".repeat(69).to_owned(), 69)
    // The method will apply one HTTP plugin and one model plugin,
    // configuring them with the input arguments. Configuration can be
    // declared to be fallible, in which case we get a `Result` we unwrap
    // here.
    .expect("failed to configure aws_auth")
    .build()
    // Since `aws_auth` has been marked as required, if the user misses
    // invoking it, this would panic here.
    .unwrap();
```

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