Let async-graphql handle cursor encoding/decoding

[msk]

async-graphql's query function automatically handles cursor encoding and decoding. This commit removes the custom cursor encoding/decoding code from the slicing module and uses async-graphql's built-in functionality instead. This change only updates the slicing module to keep the PR minimal, and a follow-up change will be needed to address the top-level encode_cursor and decode_cursor functions in the graphql module.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 14.44444% with 77 lines in your changes missing coverage. Please review.

Project coverage is 68.36%. Comparing base (d7a30a3) to head (e38c38a).

Files with missing lines	Patch %	Lines
src/graphql/statistics.rs	7.89%	35 Missing :warning:
src/graphql/slicing.rs	0.00%	19 Missing :warning:
src/graphql/model.rs	0.00%	16 Missing :warning:
src/graphql/cluster.rs	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #348 +/- ## ========================================== - Coverage 68.48% 68.36% -0.12% ========================================== Files 67 67 Lines 13560 13595 +35 ========================================== + Hits 9286 9294 +8 - Misses 4274 4301 +27 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.