Added Calls for channels.invite and users.info

[avbk]

Hey Guys,

I needed a script to invite people automatically to specific private rooms. And since I like kotlin I tried to use this library and found it very easy to use. But for my needs there were two calls missing, one to invite someone to a specific room and one to find a user by his username.

I've added both and tried to copy your style. I hope you like it :-)

Best regards
Alex

[CLAassistant]

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[avbk]

I realized that @R12rus wrote something similar in #209 and I liked his approach using RestResult instead of creating a new response class. Thus I incorporated his idea and extended my getUserByUsername() and introduced getUserById(). I also wrote some supporting tests.

I decided to consume the APIException, if the username or id is not known, and return an optional User instead, because one will write more robust code in Kotlin when using nullable types instead of exceptions.