[WIP] support external blob

[imeoer]

Related PR: containerd/nydus-snapshotter#580

Relevant Issue (if applicable)

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Details

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Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Attention: Patch coverage is 41.47627% with 333 lines in your changes missing coverage. Please review.

Project coverage is 60.22%. Comparing base (dd9ba54) to head (f914b91).
Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
builder/src/directory.rs	0.00%	84 Missing ⚠️
storage/src/backend/external/meta.rs	0.00%	40 Missing ⚠️
storage/src/backend/external/mod.rs	10.00%	36 Missing ⚠️
src/bin/nydus-image/main.rs	0.00%	35 Missing ⚠️
builder/src/core/context.rs	61.53%	18 Missing and 7 partials ⚠️
storage/src/backend/external/local.rs	0.00%	22 Missing ⚠️
builder/src/attributes.rs	75.67%	14 Missing and 4 partials ⚠️
storage/src/cache/fscache/mod.rs	33.33%	15 Missing and 1 partial ⚠️
storage/src/cache/filecache/mod.rs	31.81%	15 Missing ⚠️
builder/src/core/node.rs	57.69%	8 Missing and 3 partials ⚠️
... and 10 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1545      +/-   ##
==========================================
- Coverage   60.42%   60.22%   -0.21%     
==========================================
  Files         146      150       +4     
  Lines       48943    49398     +455     
  Branches    46380    46835     +455     
==========================================
+ Hits        29573    29748     +175     
- Misses      17906    18174     +268     
- Partials     1464     1476      +12     

Files with missing lines	Coverage Δ
api/src/config.rs	88.78% <100.00%> (+0.03%)	⬆️
builder/src/core/bootstrap.rs	77.63% <100.00%> (+0.29%)	⬆️
builder/src/core/chunk_dict.rs	70.18% <ø> (ø)
builder/src/lib.rs	64.25% <ø> (ø)
builder/src/merge.rs	70.73% <100.00%> (ø)
builder/src/stargz.rs	75.00% <100.00%> (+0.14%)	⬆️
builder/src/tarball.rs	63.26% <100.00%> (+0.57%)	⬆️
storage/src/backend/mod.rs	56.19% <ø> (ø)
storage/src/device.rs	67.83% <100.00%> (+0.10%)	⬆️
storage/src/meta/chunk_info_v1.rs	97.11% <100.00%> (ø)
... and 21 more

... and 2 files with indirect coverage changes

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[github-actions]

This PR is stale because it has been open 60 days with no activity.

[github-actions]

This PR was closed because it has been stalled for 7 days with no activity.