Add padding blob to data packs to mitigate chunking attacks

[aawsome]

see e.g. https://github.com/rustic-rs/rustic/discussions/1439 This also introduces the repository config option use-pack-padding which allows to disable the padding.

As a side-effect currently wrong statistics (data added to blobs in stats was without pack header) has been corrected.

depends on #409

[codecov[bot]]

Codecov Report

:x: Patch coverage is 64.22287% with 122 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 44.0%. Comparing base (64d052d) to head (7738b70). :warning: Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/blob/repopacker.rs	61.3%	53 Missing :warning:
crates/core/src/commands/prune.rs	64.4%	32 Missing :warning:
crates/core/src/blob/packer.rs	72.7%	12 Missing :warning:
crates/core/src/blob/pack_sizer.rs	65.0%	7 Missing :warning:
crates/core/src/commands/repair/index.rs	0.0%	6 Missing :warning:
crates/core/src/index/indexer.rs	89.2%	3 Missing :warning:
crates/core/src/backend/decrypt.rs	33.3%	2 Missing :warning:
crates/core/src/commands/copy.rs	0.0%	2 Missing :warning:
crates/core/src/commands/merge.rs	0.0%	2 Missing :warning:
crates/core/src/commands/repair/snapshots.rs	0.0%	2 Missing :warning:
... and 1 more

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/archiver/file_archiver.rs	63.1% <100.0%> (ø)
crates/core/src/archiver/tree_archiver.rs	69.7% <100.0%> (ø)
crates/core/src/backend.rs	54.6% <ø> (+1.3%)	:arrow_up:
crates/core/src/blob.rs	81.8% <ø> (+1.8%)	:arrow_up:
crates/core/src/chunker.rs	50.5% <ø> (ø)
crates/core/src/commands/config.rs	35.8% <100.0%> (+0.8%)	:arrow_up:
crates/core/src/repofile/configfile.rs	47.0% <100.0%> (-5.9%)	:arrow_down:
crates/core/src/repofile/packfile.rs	63.4% <ø> (-5.7%)	:arrow_down:
crates/core/src/archiver.rs	57.6% <66.6%> (-2.1%)	:arrow_down:
crates/core/src/backend/decrypt.rs	48.1% <33.3%> (+0.2%)	:arrow_up:
... and 9 more

... and 19 files with indirect coverage changes

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