feat: drop Link layer in favor of package:gql_link and package:gql_exec

[klavs]

BREAKING CHANGE: Link layer is now implemented via package:gql_link and package:gql_exec

First steps towards #563

• drop Link layer in favor of package:gql_link and package:gql_exec
• examples not updated as they are already outdated
• docs to be updated when more changes come in
• the surface between client and link could still get some love (better error handling)
• this change is probably not bug-free and could be improved later PRs against modularization branch
• the package:gql_*_link family is not up to par with the code removed from here, but it'll be done before modularization branch is merged. Also, that's the point of modularization - the Link layer can be improved independently from the client.

[codecov]

Codecov Report

Merging #564 into modularization will decrease coverage by 10.66%.
The diff coverage is 66.66%.

@@                 Coverage Diff                 @@
##           modularization     #564       +/-   ##
===================================================
- Coverage           59.89%   49.22%   -10.67%     
===================================================
  Files                  38       19       -19     
  Lines                1471      772      -699     
===================================================
- Hits                  881      380      -501     
+ Misses                590      392      -198

Flag	Coverage Δ
#graphql_client	50.33% <66.66%> (-10.36%)	⬇️
#graphql_flutter	22.58% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...raphql/lib/src/exceptions/operation_exception.dart	28.57% <ø> (ø)	⬆️
packages/graphql/lib/src/core/query_options.dart	43.43% <ø> (-1.02%)	⬇️
packages/graphql/lib/src/graphql_client.dart	75% <0%> (ø)	⬆️
...kages/graphql/lib/src/core/raw_operation_data.dart	71.42% <50%> (-6.84%)	⬇️
packages/graphql/lib/src/core/query_manager.dart	54.31% <76.19%> (+3%)	⬆️
...ckages/graphql/lib/src/utilities/get_from_ast.dart	50% <0%> (-50%)	⬇️

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[HofmannZ]

🎉 This PR is included in version 4.0.0-beta.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[HofmannZ]

🎉 This PR is included in version 4.0.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀