chore: add codecov.yml to ignore some dirs that don't need code coverage

[chunshao90]

Which issue does this PR close?

Closes #

Rationale for this change

Exclude some directories that don't need code coverage.

What changes are included in this PR?

Ignore following dirs in codecov:

• "benchmarks"
• "integration_test"
• "meta_client"
• "remote_engine_client"
• "wal/src/message_queue_impl"
• "wal/src/table_kv_impl"

Are there any user-facing changes?

No.

How does this change test

No need.

[codecov-commenter]

Codecov Report

Merging #591 (839e9bf) into main (6d57582) will increase coverage by 3.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #591      +/-   ##
==========================================
+ Coverage   62.61%   66.07%   +3.45%     
==========================================
  Files         326      284      -42     
  Lines       50232    43799    -6433     
==========================================
- Hits        31454    28938    -2516     
+ Misses      18778    14861    -3917     

Impacted Files	Coverage Δ
analytic_engine/src/sst/file.rs	76.17% <0.00%> (-1.09%)	⬇️
analytic_engine/src/instance/write_worker.rs	72.41% <0.00%> (-0.77%)	⬇️
analytic_engine/src/row_iter/merge.rs	88.58% <0.00%> (-0.19%)	⬇️
benchmarks/src/config.rs
wal/src/message_queue_impl/encoding.rs
benchmarks/src/merge_sst_bench.rs
wal/src/message_queue_impl/namespace.rs
wal/src/table_kv_impl/wal.rs
remote_engine_client/src/cached_router.rs
wal/src/table_kv_impl/mod.rs
... and 35 more

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[ShiKaiWi]

LGTM