CredentialsProvider class added to support password rotation

[barshaul]

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

Instead of providing a simple pair of username+password, the users can create a CredentialsProvider object with their own credentials supplier function, and have redis-py call it whenever a new connection is created.
By doing so, users will be able to fetch the current password and rotate credentials without having to create a new client.

See related feature request in Lettuce: redis/lettuce#1774

[codecov-commenter]

Codecov Report

Base: 92.04% // Head: 92.12% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (4c82551) compared to base (fb64743).
Patch coverage: 97.28% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2261      +/-   ##
==========================================
+ Coverage   92.04%   92.12%   +0.07%     
==========================================
  Files         110      113       +3     
  Lines       28746    29063     +317     
==========================================
+ Hits        26460    26773     +313     
- Misses       2286     2290       +4     

Impacted Files	Coverage Δ
redis/cluster.py	89.85% <ø> (ø)
redis/asyncio/connection.py	86.61% <84.61%> (+0.22%)	⬆️
redis/connection.py	86.42% <85.71%> (+0.11%)	⬆️
redis/credentials.py	91.66% <91.66%> (ø)
tests/test_credentials.py	98.44% <98.44%> (ø)
tests/test_asyncio/test_credentials.py	98.73% <98.73%> (ø)
redis/__init__.py	90.90% <100.00%> (+0.43%)	⬆️
redis/asyncio/client.py	92.28% <100.00%> (+0.01%)	⬆️
redis/asyncio/cluster.py	90.77% <100.00%> (+0.01%)	⬆️
redis/client.py	89.12% <100.00%> (+0.02%)	⬆️
... and 5 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[dvora-h]

Except these tiny comments - it looks great!

[barshaul]

Added type hints + moved CredentialsProvider to a separate file

[barshaul]

@chayim @dvora-h - Finished round, ready for review :). Tests failed here from some internal error, in my fork it seems that tests are passing:
barshaul#16 (comment)

[chayim]

@barshaul Thanks as always. I left a tonne of comments - and will give it another once over. As part of this, I suggest a MockCredentialProvider or similar. It would be nice to provide an example of a standard provider, perhaps implemented by a single (useless) function. This is how @dvora-h initially looked at this PR in fact.

[barshaul]

@chayim Finished round. I left some of the reviews open with questions

[barshaul]

@chayim @dvora-h @akx - finished round

[dvora-h]

@barshaul I think that after adding async support we are good to go.

[dvora-h]

LGTM now!