Add encodings to save_mfdataset

[aulemahal]

• [ ] Closes #xxxx
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

Simply add a encodings argument to save_mfdataset. As for the other args, it expects a list of dictionaries, with encoding information to be passed to to_netcdf for each dataset. Added a minimal test, simply to see if the argument was taken into account.

[github-actions[bot]]

Unit Test Results

         6 files           6 suites   53m 47s :stopwatch: 16 227 tests 14 508 :heavy_check_mark: 1 719 :zzz: 0 :x: 90 558 runs  82 397 :heavy_check_mark: 8 161 :zzz: 0 :x:

Results for commit 23acbb84.