feat(models): add service tables

[Alputer]

This PR adds Service table to db. Currently, we have only dask service but it is a generic implementation and can be used for other components associated with workflows in the future.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.13%. Comparing base (05a34ba) to head (441a391).

Files with missing lines	Patch %	Lines
reana_db/models.py	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
+ Coverage   73.76%   74.13%   +0.36%     
==========================================
  Files           7        7              
  Lines         930      951      +21     
==========================================
+ Hits          686      705      +19     
- Misses        244      246       +2     

Files with missing lines	Coverage Δ
reana_db/models.py	90.39% <90.47%> (+<0.01%)	⬆️

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