feat: Allow registering multiple functions with one server for local testing

[jihuin]

There are two ways to test multiple functions within one server.

1. Declaratively register multiple functions (e.g. calling functions.HTTP()) and run go run cmd/main.go without setting the env var FUNCTION_TARGET. In this case, all registered functions will be served at "/{func_name}".
2. Manually register the handers (e.g. calling funcframework.RegisterHTTPFunctionContext()) and run go run cmd/main.go without setting the env var FUNCTION_TARGET. In this case, the registered handlers will be served at the user defined path.

Closes #109