ENH: Refactor server handling

[isabelizimm]

Added in this PR

• removes ALL handling of data from vetiver_post
• add helper functions for data handling to new file, helpers.py
• some small refactoring of testing the API (eg, parameterizing values, removing duplicated tests in vetiver/tests/test_predict.py and vetiver/tests/test_sklearn.py)
• updates error messages to be more descriptive of location and feature causing error

previous error message:

422 Unprocessable Entity 
TypeError: Predict expects DataFrame, Series, or dict.

new error message:

1 validation error for Request: body -> 0 -> disp:   value is not a valid float (type=type_error.float)

where 0 is the index of the error and disp is the feature with the error

Related Issues

• closes Consider importing rsconnect.api.RSConnectServer #152
• closes Example dropdown improvement #85
• closes move all data preprocessing to handler #147

Notes

This PR must be merged before #143 since spacy's input data of one column of strings cannot be handled in the same way as tabular data, and the output might be multiple columns. This PR gives the flexibility of input/output handling to each modeling framework's handler.

[has2k1]

This is good to merge. The improvements can come afterwards. The important one is, full type-annotations that pass a type checker. It would make clearer how the input data, output data and api data are juggled.

[has2k1]

I think this (or immediately after this PR) is the good time to add a module docstring for all the whats and the whys about Handlers. I will do a PR after this one.

[isabelizimm]

Awesome! That would be a great addition for more context on Handlers.