Resolve "Mark kraken.spot.Staking as deprecated and add kraken.spot.Earn"

[btschwertfeger]

• The Staking endpoints were marked as deprecated and will be removed in a future release.
• The new Earn client was added with all its current endpoints.

[codecov]

Codecov Report

Attention: Patch coverage is 64.19753% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 88.41%. Comparing base (3f53ec7) to head (63918a4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
- Coverage   89.46%   88.41%   -1.06%     
==========================================
  Files          21       24       +3     
  Lines        1879     1959      +80     
==========================================
+ Hits         1681     1732      +51     
- Misses        198      227      +29     

Flag	Coverage Δ
unittests	88.41% <64.19%> (-1.06%)	⬇️

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Files	Coverage Δ
kraken/base_api/__init__.py	89.06% <ø> (ø)
kraken/exceptions/__init__.py	99.29% <100.00%> (+0.09%)	⬆️
kraken/spot/__init__.py	100.00% <100.00%> (ø)
kraken/spot/funding.py	46.57% <ø> (ø)
kraken/spot/market.py	100.00% <ø> (ø)
kraken/spot/staking.py	65.62% <100.00%> (+9.62%)	⬆️
kraken/spot/trade.py	94.33% <ø> (ø)
kraken/spot/user.py	99.12% <ø> (ø)
kraken/utils/__init__.py	100.00% <100.00%> (ø)
kraken/utils/utils.py	83.33% <83.33%> (ø)
... and 1 more