Introduce persisted queries

[williambailey]

Note: Currently implemented on top of #20 so will include some extra noise at the moment.

Takes a different approach that what I first proposed in #12.

• Deliberately kept the responsibility for performing the parsing and validating of any persisted query to PersistedQueryRetrieval implementations.
• Attempts to keep the calling side similar to the existing graphql function.

Due to the decision to put parsing and validating queries as the responsibility of any PersistedQueryRetrieval implementation we have to make a version of parse() and validate() public. However this does mean that implementations are free to decide when, and if, parsing and validation happens.

This means that an implementation is free to be able to only parse and validate a persisted query once when its first requested or at application launch. Therefore removing the parsing and validation time from responses.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.21%. The diff coverage is 85.16%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   80.04%   80.25%   +0.21%     
==========================================
  Files          74       78       +4     
  Lines        8904     9239     +335     
==========================================
+ Hits         7127     7415     +288     
- Misses       1777     1824      +47

Impacted Files	Coverage Δ
...rces/GraphQL/Instrumentation/Instrumentation.swift	100% <100%> (ø)
Sources/GraphQL/Language/Parser.swift	90.9% <100%> (+0.36%)	:arrow_up:
Sources/GraphQL/GraphQL.swift	100% <100%> (ø)	:arrow_up:
Sources/GraphQL/Validation/Validate.swift	21.71% <100%> (+2.66%)	:arrow_up:
Sources/GraphQL/Execution/Execute.swift	74.79% <75.67%> (+0.2%)	:arrow_up:
.../PersistedQueriesTests/PersistedQueriesTests.swift	78.43% <78.43%> (ø)
...entation/DispatchQueueInstrumentationWrapper.swift	81.81% <81.81%> (ø)
...ts/InstrumentationTests/InstrumentationTests.swift	89.47% <89.47%> (ø)
... and 4 more

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