issues: 473795509

In this PR, I'm experimenting with using the entrypoints package to support 3rd party backends. This does not attempt to solidify the API for what the store is, I feel like that should happen in a second PR. Here's how it would work...

In @rabernat's xmitgcm package, there is a _MDSDataStore that inherits from xarray.backends.common.AbstractDataStore. To allow reading mds datasets directly in xarray.open_dataset, xmitgcm would add the following lines to its setup.py file:

python setup( ... entry_points={ 'xarray.backends': [ 'mds = xmitgcm.mds_store:_MDSDataStore', ... ] } )

Xarray would then be able to discover this backend at runtime and users could use the store directly in open_dataset calls like this:

python ds = xr.open_dataset('./path/to/file.mds', engine='mds', backend_kwargs={...})

Note: I recognize that xmitgcm.open_mdsdataset has a bunch of other user options that I'm likely ignoring here but this is meant just as an illustration.

Now a list of caveats and things to consider:

1. I have only done this for open_dataset, not for to_netcdf. We may want to consider more generic serialization method that allows for plug-able writers.
2. open_dataset has some special handling for some readers (lock and group selection, file-like objects, etc.). We should work toward moving as much of that logic into the Store objects as possible.
3. We should decide what to do when a 3rd party plugin conflicts with an existing backend. For example, someone could include an entrypoint with the key of netcdf4.

• [x] Partially closes #1970
• [ ] Tests added
• [ ] Fully documented, including whats-new.rst for all changes and api.rst for new API