Builtin endpoints do not follow CRUD

[jonathanhliu21]

get() and receive() and other endpoints which get data from serial port should only be accessible via GET requests, and sending data should be accessible via POST requests.

Additionally, the names/terms of some RestApiHandler methods, such as add_endpoint(), are wrong, because it is actually adding a resource. Endpoints are different for different request methods. The method/decorator should be called something like add_conn_resource() instead.

[jonathanhliu21]

This isn't breaking change because we can create an alias for the add_endpoint() method but rename it to something else. Additionally, there is API versioning.

[jonathanhliu21]

I will fix the second part of this issue later because there are too many places where "route", "endpoint", and "resource" are used interchangeably. This includes the documentation, comments in the code, variable names, method names, parameter names, etc.