nydusify: introduce chunkdict generate subcommand

[cslinwang]

Brief introduction
Add functionlity to generate chunk dictionary by database file and algorithm "exponential_smoothing"
Implement the command nydus-image chunkdict generate --database command

Basic Usage
nydus-image chunkdict generate --database ./metadata.db

Details
Write command “nydus-image chunkdict generate” entries
Call the database interface and get the chunk information in the database
Implement the exponential smoothing algorithm and obtain chunk dictionary
To be continued, dump chunk dictionary and save, and then compact chunk

[codecov]

Codecov Report

Attention: Patch coverage is 54.95557% with 659 lines in your changes are missing coverage. Please review.

Project coverage is 61.42%. Comparing base (e926d2f) to head (6f58336).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1572      +/-   ##
==========================================
+ Coverage   61.23%   61.42%   +0.18%     
==========================================
  Files         144      145       +1     
  Lines       47100    47984     +884     
  Branches    44602    45960    +1358     
==========================================
+ Hits        28843    29474     +631     
- Misses      16778    16951     +173     
- Partials     1479     1559      +80     

Files	Coverage Δ
builder/src/lib.rs	64.66% <ø> (ø)
rafs/src/metadata/cached_v5.rs	80.79% <100.00%> (+0.02%)	⬆️
rafs/src/metadata/layout/v6.rs	85.23% <100.00%> (+0.04%)	⬆️
storage/src/cache/filecache/mod.rs	67.25% <50.00%> (-0.25%)	⬇️
storage/src/meta/mod.rs	72.51% <89.47%> (+0.12%)	⬆️
rafs/src/metadata/layout/v5.rs	85.03% <25.00%> (-0.21%)	⬇️
storage/src/device.rs	69.20% <62.50%> (-0.06%)	⬇️
utils/src/digest.rs	88.32% <0.00%> (-3.74%)	⬇️
contrib/nydusify/cmd/nydusify.go	13.42% <0.00%> (-0.91%)	⬇️
builder/src/core/context.rs	65.72% <22.58%> (-1.35%)	⬇️
... and 3 more

... and 17 files with indirect coverage changes

[imeoer]

Thanks for the PR, let's add SOB for all commits to pass https://github.com/dragonflyoss/nydus/pull/1572/checks?check_run_id=24019241046.