feat: Add CSV export capability to DataFrameClient

[kjohn1922]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Add export_table_data to the DataFrameClient, which calls the export route on the DataFrame Service API.

Wrap the response.iter_content generator in a file-like object that can either be written to a file or loaded into a pandas dataframe, without loading the data into memory all at once beforehand.

Added an example of writing the export data to a file or pandas.

Why should this Pull Request be merged?

Parity of the python API with new features added to the DataFrame Service API.

What testing has been done?

Manual testing using Jupyter notebooks. Added unit tests for the generator wrapper class. Added an integration test for the export data method.

[kjohn1922]

A couple of comments on this:

• It's returning the data as binary data. The binary can be written to a CSV file or to pandas directly, but if the user wants to get the data as a string in code, they will need to decode it themselves. We did this in anticipation of adding binary export formats in the future (TDMS).
• I didn't implement readline or readlines, since they are not required for writing to a file or to pandas. I can add them if we want this to be a fully-functional file-like object, but they were getting a bit messy with iterating the binary and normalizing line endings to what python expects.

[kjohn1922]

I'm working through some type errors.

[kjohn1922]

This was required to satisfy the mypy type checker for having pandas in the export-data example. mypy needs pandas-stubs to do type checking for pandas, and pandas-stubs requires python to be between these two versions.

If we're not happy with this, I can remove pandas-stubs and revert this line, because it just gives a warning from mypy that it's skipping type checking for pandas without it.

[mure]

From https://dev.azure.com/ni/DevCentral/_workitems/edit/2347097,

If the server terminates the connection for any reason, the client will get an unhelpful requests.exceptions.ChunkedEncodingError error. It would be nice to catch this and have a more user friendly message.

[kjohn1922]

From https://dev.azure.com/ni/DevCentral/_workitems/edit/2347097,

If the server terminates the connection for any reason, the client will get an unhelpful requests.exceptions.ChunkedEncodingError error. It would be nice to catch this and have a more user friendly message.

We have a couple of ideas for how to solve this, a couple of which would be changes on the DFS side. I added some details to the bug, and we can decide on the approach when we get to it.