feat: add additional imports

[skonik]

This PR allows to pass additional imports within generate function.

The problem I was facing was that I needed to add custom classes for models generated inside output file.
However, adding new classes to custom template didn't affect output file at all. The reason was how imports are added to rendered data model template. This PR solves such kind of issue.

This allows me to do such things:

1. Specify some kind of python object inside template(BaseModel in my case)

2. Call generate function with custom base_class and aditional_imports with pydantic BaseModel

3. As a result generated code has required imports.

Waiting for feedback regarding the feature and the code itself.

Closes #1421

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (6225b8f) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1422     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        32     +21     
  Lines         1020      3547   +2527     
  Branches       201       833    +632     
===========================================
+ Hits          1020      3547   +2527     

Flag	Coverage Δ
unittests	99.63% <99.57%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/improts.py	100.00% <100.00%> (ø)
... and 22 more

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[koxudaxi]

@skonik
Sorry for the delay in thanking you.
Thank you for your contribution!