Make 'meta' command capable of editing attributes

[dmach]

Fixes: #435

[dmach]

Related to openSUSE/open-build-service#13938 - it may allow us dropping the new code doing POST request instead of PUT

[codecov]

Codecov Report

Patch coverage: 21.03% and project coverage change: +0.24% 🎉

Comparison is base (3559336) 28.98% compared to head (95998ef) 29.22%.
Report is 37 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1360      +/-   ##
==========================================
+ Coverage   28.98%   29.22%   +0.24%     
==========================================
  Files          36       47      +11     
  Lines       16729    17238     +509     
==========================================
+ Hits         4849     5038     +189     
- Misses      11880    12200     +320     

Files Changed	Coverage Δ
osc/_private/project.py	0.00% <0.00%> (ø)
osc/commandline.py	19.08% <0.00%> (+0.06%)	⬆️
osc/commands/repo.py	0.00% <0.00%> (ø)
osc/commands/repo_add.py	0.00% <0.00%> (ø)
osc/commands/repo_list.py	0.00% <0.00%> (ø)
osc/commands/repo_remove.py	0.00% <0.00%> (ø)
osc/_private/api.py	46.66% <8.00%> (-14.88%)	⬇️
osc/core.py	35.30% <11.11%> (-0.35%)	⬇️
osc/output/tty.py	46.66% <46.66%> (ø)
osc/output/key_value_table.py	86.79% <86.79%> (ø)
... and 3 more

... and 8 files with indirect coverage changes

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