feat: add client methods to read/update metadata

[IgnacioHeredia]

This has been tested:

r = Nomad.client.metadata.read_metadata("2c73d84b-305c-d0b6-5477-57be1cf1bff5")
r = Nomad.client.metadata.update_metadata("2c73d84b-305c-d0b6-5477-57be1cf1bff5", {"status": "error"})

[codecov]

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.37%. Comparing base (41df869) to head (4ac47b8).
⚠️ Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
nomad/api/client.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   90.71%   90.37%   -0.35%     
==========================================
  Files          31       31              
  Lines        1293     1309      +16     
==========================================
+ Hits         1173     1183      +10     
- Misses        120      126       +6     

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