nydusify: introduce chunkdict generate subcommand

[newthifans]

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

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 54.43466% with 673 lines in your changes are missing coverage. Please review.

Project coverage is 61.24%. Comparing base (a0ec880) to head (6093309).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1401      +/-   ##
==========================================
- Coverage   61.34%   61.24%   -0.11%     
==========================================
  Files         144      145       +1     
  Lines       47001    48383    +1382     
  Branches    44537    45906    +1369     
==========================================
+ Hits        28835    29630     +795     
- Misses      16685    17194     +509     
- Partials     1481     1559      +78     

Files	Coverage Δ
builder/src/lib.rs	64.66% <ø> (ø)
rafs/src/metadata/cached_v5.rs	80.79% <100.00%> (+0.02%)	:arrow_up:
rafs/src/metadata/layout/v6.rs	85.23% <100.00%> (+0.04%)	:arrow_up:
storage/src/cache/filecache/mod.rs	67.25% <50.00%> (-0.25%)	:arrow_down:
storage/src/meta/mod.rs	72.51% <89.47%> (+0.12%)	:arrow_up:
rafs/src/metadata/layout/v5.rs	85.03% <25.00%> (-0.21%)	:arrow_down:
storage/src/device.rs	68.98% <62.50%> (-0.06%)	:arrow_down:
utils/src/digest.rs	88.32% <0.00%> (-3.74%)	:arrow_down:
contrib/nydusify/cmd/nydusify.go	14.79% <0.00%> (-0.31%)	:arrow_down:
builder/src/core/context.rs	65.94% <25.92%> (-1.09%)	:arrow_down:
... and 3 more

... and 2 files with indirect coverage changes

[imeoer]

@newthifans @cslinwang Please resolve the conflicts.