Improve extensibility of gRPC invocation

[JamesNK]

Public changes:

• Add new extensibility for registering a request delegate for a gRPC method

/// <summary>
/// Adds a method to a service. This method is handled by the specified <see cref="RequestDelegate"/>.
/// </summary>
/// <typeparam name="TRequest">Request message type for this method.</typeparam>
/// <typeparam name="TResponse">Response message type for this method.</typeparam>
/// <param name="method">The method description.</param>
/// <param name="pattern">The method pattern. This pattern is used by routing to match the method to an HTTP request.</param>
/// <param name="metadata">The method metadata. This metadata can be used by routing and middleware when invoking a gRPC method.</param>
/// <param name="invoker">The <see cref="RequestDelegate"/> that is executed when the method is called.</param>
public void AddMethod<TRequest, TResponse>(Method<TRequest, TResponse> method, RoutePattern pattern, IList<object> metadata, RequestDelegate invoker)
    where TRequest : class
    where TResponse : class
{
    var methodModel = new MethodModel(method, pattern, metadata, invoker);
    Methods.Add(methodModel);
}

Internal changes:

• Add public gRPC method invokers. These handle service lifetime and interceptor pipeline. Can be used by default gRPC implementation, and gRPC+HTTPAPI and gRPC-Web. Update: Now internal and shared source. We can decide whether to make public in the future if there demand.
• Add MethodOptions, an immutable view of the settings used to invoke a gRPC method (max request size, interceptors, compression). These are created from a collection of GrpcServiceOptions (mutable version). Lays the groundwork for per-method customization of settings.. Update: Now internal and shared source. We can decide whether to make public in the future.

[JamesNK]

I've greatly reduced the new API surface. Now there is just one new public method. Some code has moved to a shared source folder. If we ever decided to not share it then its simple to move it back again.

Ready for review @jtattermusch

[jtattermusch]

Can you give a list of publicly visible changes? I went through the changes and noticed that besides, the new AddMethod overload you're also publishing IGrpcInterceptorActivator and IGrpcInterceptorActivator<TInterceptor> etc., so I wanted to make sure I didn't miss anything.

[jtattermusch]

Overall looks lood, added a few questions. I'd also like a detailed review from @JunTaoLuo

[jtattermusch]

Are the changes from #677 required for this PR? If not, I'd prefer merging the two separately.

[JamesNK]

Are the changes from #677 required for this PR? If not, I'd prefer merging the two separately.

It will be merged before this one.

Edit: #677 is merged.

[JunTaoLuo]

Some minor nits.

[jtattermusch]

LGTM.