Feature: support static config for Semian::NetHTTP and Semian::GRPC

Background

MySQL and Redis supports static config, while NetHTTP and GRPC currently only Proc. e.g.
https://github.com/Shopify/semian/blob/1d4b53be7819707f9c6eebc17bb9b50b46619271/lib/semian/mysql2.rb#L137-L140
https://github.com/Shopify/semian/blob/1d4b53be7819707f9c6eebc17bb9b50b46619271/lib/semian/net_http.rb#L67-L69
https://github.com/Shopify/semian/blob/1d4b53be7819707f9c6eebc17bb9b50b46619271/lib/semian/grpc.rb#L48-L50

Problem

If pass a static config to NetHTTP and GRPC, semian is disabled because retrieve_semian_configuration returns nil.

Proposal

allow static config if it is not callable

def retrieve_semian_configuration(host, port)
  return @semian_configuration unless @semian_configuration.respond_to?(:call) 

 @semian_configuration.call(host, port)
 end 

Alternative

Add validation to semian_configuration setter, assert whether static config or Proc is expected

https://github.com/Shopify/semian/blob/1d4b53be7819707f9c6eebc17bb9b50b46619271/lib/semian/net_http.rb#L61-L64

Start with lib/semian/net_http.rb and lib/semian/grpc.rb, especially the semian_configuration setter and retrieve_semian_configuration methods; compare their behavior with the referenced implementation in lib/semian/mysql2.rb. Verify that static configurations are returned for both integrations, callable configurations still receive host and port, and the relevant existing tests pass.