Fetchmore, json encoding, and query var change patches

[micimize]

closes #389, #386, #374, #351

Fixes / Enhancements

• default to object.toJson() in toKey() (Crash serializing json_serializable objects #374)
• use noCache when calling fetchMore to avoid cache writes/normalization
• add mechanism to writeToCache in addQueryResult so fetchMore now calls:

      queryManager.addQueryResult(
        queryId,
        fetchMoreResult,
        writeToCache: true,
      );

• Added the eager result to the stream onListen (QueryResult loading not 'true' when changing variables (in QueryOptions) #351 (comment))
• Added a key to Query` to reset the stream (initial renders with new variables would have stale data otherwise)

[mainawycliffe]

LGTM

[codecov]

Codecov Report

Merging #394 into beta will decrease coverage by 0.39%.
The diff coverage is 8.33%.

@@            Coverage Diff            @@
##             beta     #394     +/-   ##
=========================================
- Coverage   58.04%   57.64%   -0.4%     
=========================================
  Files          32       32             
  Lines        1299     1308      +9     
=========================================
  Hits          754      754             
- Misses        545      554      +9

Flag	Coverage Δ
#graphql_client	58.33% <8.33%> (-0.42%)	⬇️
#graphql_flutter	29.03% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...kages/graphql/lib/src/core/raw_operation_data.dart	94.11% <0%> (-5.89%)	⬇️
...ackages/graphql/lib/src/core/observable_query.dart	6.92% <0%> (-0.28%)	⬇️
packages/graphql/lib/src/core/query_manager.dart	48.3% <25%> (-1.27%)	⬇️

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[HofmannZ]

🎉 This PR is included in version 2.1.1-beta.1 🎉

The release is available on GitHub release

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[HofmannZ]

🎉 This PR is included in version 3.0.0-beta.1 🎉

The release is available on GitHub release

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[HofmannZ]

🎉 This PR is included in version 3.0.0 🎉

The release is available on GitHub release

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