Add type hints

[metaodi]

Type hints are here to stay and improve the developer experience. By providing hints to all functions, the use of osmapi should be easier for developers.

And static code analyzers could help find bugs.

[susrisha]

@metaodi is it taken care of or you need help??

[metaodi]

@susrisha I have already a local branch with some work on this. But I'm not sure yet how this will impact the API regarding #39, it's still a lot of uncertaintly around this issue.