Async Connection: Allow PubSub.run() without previous subscribe()

[kristjanvalur]

Pull Request check-list

• 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?

Description of change

This change adds a "connect()" method to the PubSub class and employs that in the run method.
This makes it possible to start a run() task without knowing beforehand which subscriptions are required,
for example, when running a server that dynamically listens to various channels.

PR is provided as a suggested feature, and so there is no documentation or non-async implementation yet.

[codecov-commenter]

Codecov Report

Merging #2148 (9543a59) into master (6ba4641) will decrease coverage by 0.07%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #2148      +/-   ##
==========================================
- Coverage   92.44%   92.37%   -0.08%     
==========================================
  Files         104      104              
  Lines       24385    24458      +73     
==========================================
+ Hits        22542    22592      +50     
- Misses       1843     1866      +23     

Impacted Files	Coverage Δ
tests/test_asyncio/test_pubsub.py	99.34% <90.47%> (-0.43%)	⬇️
redis/asyncio/client.py	91.98% <100.00%> (+0.05%)	⬆️
tests/test_commands.py	93.71% <0.00%> (-0.61%)	⬇️
redis/commands/core.py	83.78% <0.00%> (-0.23%)	⬇️
redis/client.py	89.46% <0.00%> (ø)
redis/asyncio/connection.py	84.72% <0.00%> (ø)
tests/test_cluster.py	97.60% <0.00%> (+0.23%)	⬆️
tests/conftest.py	87.25% <0.00%> (+0.79%)	⬆️
tests/test_function.py	27.58% <0.00%> (+2.28%)	⬆️

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[chayim]

Thanks @kristjanvalur! This really helps bring more parity to the async connection!

[kristjanvalur]

Well, that was fast :)
If accepted, I was planning to add some docs, and provide a "sync" implementation too. Should I do that in a separate PR?