[#12] Add producttype parameters

[Floris272]

Fixes #12

Changes

• Adds Parameter model with fk to ProductType. (very similar to the recently added model ExterneCode)
• model is an key value pair and is created within the ProductType post request.
• the filter ?parameter=[key:value].

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.64286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.70%. Comparing base (b42b561) to head (1e732cb).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/open_producten/producttypen/views.py	80.00%	1 Missing and 1 partial ⚠️
..._producten/producttypen/serializers/producttype.py	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   85.48%   85.70%   +0.21%     
==========================================
  Files         109      112       +3     
  Lines        2343     2399      +56     
  Branches      154      157       +3     
==========================================
+ Hits         2003     2056      +53     
- Misses        316      317       +1     
- Partials       24       26       +2     

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