Add interfaces

[rafalp]

This PR adds Interface type to Ariadne. Interface acts like the union, except it enforces intersection between implementing types. It also allows (optional) resolvers reuse between implementing types.

Currently, this is a proof-of-concept implementation.

TODO

• Use interface to reuse resolvers
• Use interface as return type (needs type_resolver like union does)
• Tests
• Docs
• Changelog

[codecov]

Codecov Report

Merging #112 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #112    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          41     43     +2     
  Lines        1360   1501   +141     
======================================
+ Hits         1360   1501   +141

Impacted Files	Coverage Δ
tests/test_unions.py	100% <100%> (ø)	⬆️
ariadne/__init__.py	100% <100%> (ø)	⬆️
ariadne/interfaces.py	100% <100%> (ø)
ariadne/resolvers.py	100% <100%> (ø)	⬆️
tests/test_interfaces.py	100% <100%> (ø)

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[rafalp]

@salwator I've made another pass over the docs. Could you give it a look and let me know what you think about it now?