Allow adding `ServerInterceptor`s to specific services and methods

[gjcairo]

Motivation

We want to allow users to customise the RPCs a registered interceptor should apply to on the server:

• Intercept all requests
• Intercept requests only meant for specific services
• Intercept requests only meant for specific methods

Modifications

This PR adds a new ServerInterceptorTarget type that allows users to specify what the target of the interceptor should be. Existing APIs accepting [any ServerInterceptor] have been changed to instead take [ServerInterceptorTarget].

Result

Users can have more control over to which requests interceptors are applied.