feat(inverted_index): get memory usage of appliers

[zhongzc]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Add memory_usage to traits FstApplier and IndexApplier.

It helps track the memory usage of the index application process.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b9302e4) 85.58% compared to head (01d2afa) 85.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3081      +/-   ##
==========================================
- Coverage   85.58%   85.05%   -0.53%     
==========================================
  Files         805      805              
  Lines      130658   130740      +82     
==========================================
- Hits       111822   111206     -616     
- Misses      18836    19534     +698     

[killme2008]

LGTM