Add client sessions

[fjarri]

• Added client session support
• Merged SigningClient into Client (having two types only complicates things; just taking a signer argument instead)
• Made some improvements to typing

[codecov-commenter]

Codecov Report

Merging #1 (79d4d1d) into master (126c3f5) will decrease coverage by 0.10%.
The diff coverage is 84.74%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
- Coverage   77.65%   77.55%   -0.11%     
==========================================
  Files           8        8              
  Lines         470      490      +20     
==========================================
+ Hits          365      380      +15     
- Misses        105      110       +5     

Impacted Files	Coverage Δ
pons/types.py	83.17% <0.00%> (ø)
pons/provider.py	51.42% <66.66%> (+10.51%)	⬆️
pons/client.py	91.37% <94.44%> (-0.37%)	⬇️
pons/__init__.py	100.00% <100.00%> (ø)
pons/signer.py	89.47% <100.00%> (ø)

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