[proposal]: add support for custom path methods (google aip-136)

[ryanfkeepers]

AIP-136 documentation

Proposal

Include support for custom path methods either through standard path declaration (example 1) or subroute declaration (example 2)

Examples

1 - standard path declaration

r.Post("/foo/{fooID}:bar", fooBarHandler)

2 - subroute declaration

r.Route("/foo/{fooID}", func(r chi.Router) {
    r.Post(":bar", fooBarHandler)
})