Add support to use certificates from string in ssl connection

[dvora-h]

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Description of change

closes #2047

[codecov-commenter]

Codecov Report

Merging #2048 (b8383b3) into master (8d949a3) will decrease coverage by 0.08%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master    #2048      +/-   ##
==========================================
- Coverage   92.17%   92.09%   -0.09%     
==========================================
  Files         100      100              
  Lines       21024    21064      +40     
==========================================
+ Hits        19378    19398      +20     
- Misses       1646     1666      +20     

Impacted Files	Coverage Δ
redis/asyncio/client.py	90.45% <ø> (-0.18%)	⬇️
redis/client.py	89.39% <ø> (ø)
redis/cluster.py	91.93% <ø> (ø)
redis/asyncio/connection.py	82.63% <50.00%> (-0.02%)	⬇️
redis/connection.py	86.53% <100.00%> (+0.01%)	⬆️
tests/test_ssl.py	49.64% <100.00%> (+2.63%)	⬆️
redis/commands/cluster.py	92.68% <0.00%> (-2.19%)	⬇️
tests/test_cluster.py	97.38% <0.00%> (-0.92%)	⬇️
... and 2 more

---

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[chayim]

Nit: I don't think you need to rename the test. But this PR is great. Approved.