Support customizing the Faraday client in MCP::Client::HTTP

## Motivation and Context

`MCP::Client::HTTP` builds its Faraday connection internally in a private `client` method
with no way to customize the middleware stack or adapter.
Users who need observability (request/response recording, failure logging) or
a different HTTP adapter must override a private method, coupling to an internal API.

This accepts an optional block in `MCP::Client::HTTP.new` that yields the Faraday builder
after default middleware is configured, allowing users to add custom middleware or swap
the HTTP adapter.

## How Has This Been Tested?

Added a test that verifies custom headers set via the block are included in requests.
All existing tests continue to pass.

## Breaking Changes

None. The block argument is optional, so existing code is unaffected.

Resolves #303

lib/mcp/client/http.rb

@@ -7,9 +7,10 @@ module MCP
 
       attr_reader :url
 
-      def initialize(url:, headers: {})
+      def initialize(url:, headers: {}, &block)
         @url = url
         @headers = headers
+        @faraday_customizer = block
       end
 
       def send_request(request:)
@@ -78,6 +79,8 @@ module MCP
           headers.each do |key, value|
             faraday.headers[key] = value
           end
+
+          @faraday_customizer&.call(faraday)
         end
       end