[aawsome]

This PR refactors the packer and the way packfiles are handled in rustic:

• Introduces and uses a BasicPacker which only provides the logic without doing any I/O itself
• Packfiles are no longer collected as a single BytesMut in memory, but as BytesList which equals Vec<Bytes>. This reduces allocations and unnecessary memory copies (but is a breaking change w.r.t the backend API).
• fixes the memory allocation used for the pack header. By accident, here again the total pack size was allocated. This fix should reduce the total needed memory especially when using large packfiles
• this PR also fixes some new clippy lints (according to the given suggestions)

[codecov]

Codecov Report

❌ Patch coverage is 72.26277% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.7%. Comparing base (82f32a4) to head (8cf2d88).

Files with missing lines	Patch %	Lines
crates/core/src/blob/packer.rs	77.1%	13 Missing ⚠️
crates/backend/src/rest.rs	0.0%	9 Missing ⚠️
crates/core/src/backend.rs	73.6%	5 Missing ⚠️
crates/core/src/blob/tree.rs	76.4%	4 Missing ⚠️
crates/backend/src/opendal.rs	0.0%	3 Missing ⚠️
crates/backend/src/rclone.rs	0.0%	1 Missing ⚠️
crates/core/src/backend/cache.rs	92.3%	1 Missing ⚠️
crates/core/src/backend/dry_run.rs	50.0%	1 Missing ⚠️
crates/core/src/backend/warm_up.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/backend/src/local.rs	41.4% <100.0%> (-1.1%)	⬇️
crates/core/src/backend/decrypt.rs	48.8% <100.0%> (+0.7%)	⬆️
crates/core/src/backend/hotcold.rs	57.1% <100.0%> (-7.2%)	⬇️
crates/core/src/commands/repair/hotcold.rs	65.1% <100.0%> (-1.2%)	⬇️
crates/core/src/crypto/aespoly1305.rs	85.7% <ø> (ø)
crates/core/src/repofile/packfile.rs	65.7% <100.0%> (-5.3%)	⬇️
crates/backend/src/rclone.rs	8.7% <0.0%> (ø)
crates/core/src/backend/cache.rs	57.1% <92.3%> (-1.8%)	⬇️
crates/core/src/backend/dry_run.rs	11.1% <50.0%> (ø)
crates/core/src/backend/warm_up.rs	0.0% <0.0%> (ø)
... and 5 more

... and 35 files with indirect coverage changes

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