feat(2639): implement entity resolver

[meskill]

Summary:

Implements Query._entities handler according to the doc

Please, note, I didn't add any special flags to the server and enable entity resolver if there any resolver on type. async_graphql enables the federation support either by flag or if the entity defined. docs

Caveats for current implementation:

• @link operator conflicts with apollo federation's @link. To address this I'm disabling .add_directive(Link::directive_definition(generated_types)) in the config.rs
• to be able to load the subgraph to apollo we need to provide all of our internal definitions. It's done for now explicitly in the _service handler. But the recommended approach from apollo is to load .graphql schema file and we need to provide the definitions in that case somehow
• tailcall doesn't output to sdl unknown directives but that could be useful in case of apollo federation since user may want to use apollo directives to speficy loading logic.

Issue Reference(s):
Fixes #2639
/claim #2639

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

Attention: Patch coverage is 93.70315% with 42 lines in your changes missing coverage. Please review.

Project coverage is 87.83%. Comparing base (b0bcfcf) to head (866bc3d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/blueprint/operators/apollo_federation.rs	86.99%	16 Missing ⚠️
src/core/config/resolver.rs	33.33%	10 Missing ⚠️
src/core/config/transformer/subgraph.rs	97.85%	8 Missing ⚠️
src/core/valid/valid.rs	50.00%	3 Missing ⚠️
src/core/config/directives/federation.rs	0.00%	1 Missing ⚠️
src/core/config/directives/graphql.rs	75.00%	1 Missing ⚠️
src/core/config/directives/grpc.rs	75.00%	1 Missing ⚠️
src/core/config/directives/http.rs	75.00%	1 Missing ⚠️
src/core/ir/error.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2693      +/-   ##
==========================================
+ Coverage   87.15%   87.83%   +0.67%     
==========================================
  Files         253      263      +10     
  Lines       25585    26180     +595     
==========================================
+ Hits        22299    22994     +695     
+ Misses       3286     3186     -100     

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