refactor(config): represent field resolver as separate enum

[meskill]

Summary:
Represent resolver for field as separate enum.

Pros of doing this:

• resolver now could be reused for types without copy-pasting
• single place to manage resolvers and avoid errors when adding new resolvers
• fixes present minor issues for resolvers check across the codebase

Issue Reference(s):
Fixes #... (Replace "..." with the issue number)

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 93.63057% with 10 lines in your changes missing coverage. Please review.

Project coverage is 86.26%. Comparing base (bb9fc87) to head (7f6aa6c).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/core/config/config.rs	87.50%	8 Missing ⚠️
src/cli/tc/init.rs	0.00%	1 Missing ⚠️
tailcall-typedefs/src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2675      +/-   ##
==========================================
- Coverage   86.32%   86.26%   -0.06%     
==========================================
  Files         255      255              
  Lines       24673    24670       -3     
==========================================
- Hits        21298    21282      -16     
- Misses       3375     3388      +13     

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[karatakis]

Everything okay to me.

[tusharmath]

The generated schema shouldn't have changed ideally.

[meskill]

The generated schema shouldn't have changed ideally.

This is a pure json schema issue. See GREsau/schemars#317. The actual parsing and graphql schema work as expected.