add channels and methods

[wanderingbort]

This PR adds the concept of Channels and Methods to appbase.

Channels

Channels are a means to publish data from one plugin that is consumed by many plugins without having to tightly couple the publisher plugin to the consumer plugin. This is a generic utility that encapsulates a common use case we were seeing in appbase applications where one plugin would directly expose boost signals for other plugins to subscribe to.

struct foo {
   int bar; 
};
using foo_channel_decl = channel_decl<struct my_tag, foo>;

// subscribe
auto subscription = app().get_channel<foo_channel_decl>().subscribe([](const foo& f){
   std::cout << "bar: " << f.bar << std::endl;
});

// send some foos
auto& foo_channel = app().get_channel<foo_channel_decl>();
foo_channel.publish(foo{5});  /// prints "bar: 5"
foo_channel.publish(foo{9});  /// prints "bar: 9"

Methods

Methods are a means to fetch values from or run a given command on provider plugin(s) without having to tightly couple the calling plugin to the provider plugin.

using add_method_decl = method_decl(struct my_tag, double(double,double));

// provide
auto provider = app().get_method<add_method_decl>().register_provider([](double a, double b) -> double {
   return a + b;
});

// call the method
double res;
auto& add_method = app().get_method<add_method_decl>();
res = add_method(1.0, 2.0);  /// res ~== 3.0
res = add_method(5.0, 4.0); /// res ~== 9.0