Custom authentication provider for barrel_mcp.

Allows using a custom module for authentication without implementing the full barrel_mcp_auth behaviour. The custom module only needs to export two functions:

• init(Opts) -> {ok, State} - Initialize auth state
• authenticate(Token, State) -> {ok, AuthInfo, State} | {error, Reason, State}

Usage

   barrel_mcp:start_http(#{
       port => 9090,
       auth => #{
           provider => barrel_mcp_auth_custom,
           provider_opts => #{
               module => my_auth_module
           }
       }
   }).

The custom module:

   -module(my_auth_module).
   -export([init/1, authenticate/2]).
  
   init(_Opts) ->
       {ok, #{}}.
  
   authenticate(Token, State) ->
       case validate_token(Token) of
           {ok, Info} -> {ok, Info, State};
           error -> {error, invalid_token, State}
       end.

The state returned by authenticate/2 is discarded: the provider is called with the state init/1 produced on every request, so anything that has to persist across requests belongs in a process or table of the module's own.