Intorduce property_field decorator and fix for pydantic_only plus optimization

[collerek]

• Performance optimization
• Fix for bug with pydantic_only fields being required
• Add property_field decorator that registers a function as a property that will
  be included in Model.dict() and in fastapi response
• Update docs

[codecov-io]

Codecov Report

Merging #64 (f071d45) into master (40254b9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master       #64    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           65        69     +4     
  Lines         5747      5902   +155     
==========================================
+ Hits          5747      5902   +155     

Impacted Files	Coverage Δ
ormar/__init__.py	100.00% <100.00%> (ø)
ormar/decorators/__init__.py	100.00% <100.00%> (ø)
ormar/decorators/property_field.py	100.00% <100.00%> (ø)
ormar/fields/model_fields.py	100.00% <100.00%> (ø)
ormar/models/metaclass.py	100.00% <100.00%> (ø)
ormar/models/modelproxy.py	100.00% <100.00%> (ø)
ormar/models/newbasemodel.py	100.00% <100.00%> (ø)
ormar/models/quick_access_views.py	100.00% <100.00%> (ø)
tests/test_excluding_fields_in_fastapi.py	100.00% <100.00%> (ø)
tests/test_properties.py	100.00% <100.00%> (ø)
... and 5 more