Allow injecting methods with generic type parameters in the config object (#3274)

This is a follow-up to #3111. Currently, the injected methods are
limited to taking in concrete types. This PR allows for these methods to
take in generic type parameters as well.

```rust
impl<L, H, M> SimpleServiceConfigBuilder<L, H, M> {
    pub fn aws_auth<C>(config: C) {
        ...
    }
}
```

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