DSL meta fields implementation

[leszekhanusz]

Allow to add meta-fields (__typename, __schema and __type) to a DSL schema.

Implementation of get_introspection_query_ast method which creates the introspection query using the DSL module with a configurable level of recursion.

[codecov-commenter]

Codecov Report

Merging #259 (e2a69a1) into master (6b53422) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #259   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        22    +1     
  Lines         1662      1741   +79     
=========================================
+ Hits          1662      1741   +79     

Impacted Files	Coverage Δ
gql/dsl.py	100.00% <100.00%> (ø)
gql/utilities/__init__.py	100.00% <100.00%> (ø)
gql/utilities/get_introspection_query_ast.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6b53422...e2a69a1. Read the comment docs.