feat(playground): GraphiQL sandbox. Allow using the cube GraphQL API

[vasilev-alex]

No description provided.

[codecov]

Codecov Report

Merging #3803 (2144136) into master (ea9630f) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3803      +/-   ##
==========================================
+ Coverage   57.62%   57.66%   +0.04%     
==========================================
  Files         126      126              
  Lines       10564    10564              
  Branches     2497     2500       +3     
==========================================
+ Hits         6087     6092       +5     
+ Misses       4194     4189       -5     
  Partials      283      283              

Impacted Files	Coverage Δ
packages/cubejs-api-gateway/src/gateway.ts	67.35% <0.00%> (ø)
packages/cubejs-api-gateway/src/graphql.ts	4.25% <0.00%> (ø)
packages/cubejs-api-gateway/src/sql-server.ts	6.89% <0.00%> (ø)
...kages/cubejs-api-gateway/src/SubscriptionServer.ts	7.69% <0.00%> (ø)
...es/cubejs-schema-compiler/src/adapter/BaseQuery.js	64.07% <0.00%> (+0.41%)	⬆️

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