Added ability to explicitly pass input and output types to DjangoSerializerMutation

[bloodwithmilk25]

There are cases(I had it), when users want explicitly pass input/output types to DjangoSerializerMutation. There may be a conflict with my other pull request, as it's the same file, I will help you resolve it.

[codecov[bot]]

Codecov Report

Merging #157 into master will decrease coverage by 0.05%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   53.65%   53.60%   -0.06%     
==========================================
  Files          24       24              
  Lines        1968     1970       +2     
==========================================
  Hits         1056     1056              
- Misses        912      914       +2     

Impacted Files	Coverage Δ
graphene_django_extras/mutation.py	32.07% <0.00%> (-0.41%)	:arrow_down:

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[tvanmaren]

This would be very useful, particularly with JSON data fields, which currently get auto-stringified and stuck as non-sub-selectable (which creates an asymmetry between query & mutation types that then has to be hack-fixed by the client).