Adding MEXISTS to CFCommands

[jspalink]

Adding an implementation of MEXISTS to the list of CFCommands. This code is a barely edited copy of the corresponding Bloom Filter command.

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

Adding a MEXISTS command for Cuckoo Filters.

[chayim]

@jspalink Thanks for the change! Do you think you could add a unit test as well?

[chayim]

@dvora-h Perhaps you can make sure we're testing properly here?

[codecov-commenter]

Codecov Report

Merging #2090 (c591e2b) into master (e5e265d) will decrease coverage by 0.01%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #2090      +/-   ##
==========================================
- Coverage   92.44%   92.43%   -0.02%     
==========================================
  Files         104      104              
  Lines       24364    24369       +5     
==========================================
+ Hits        22524    22526       +2     
- Misses       1840     1843       +3     

Impacted Files	Coverage Δ
redis/commands/bf/commands.py	96.61% <40.00%> (-1.41%)	⬇️

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[dvora-h]

Duplicate, the command was added in #2184