feat!: Add support for multiple parents

[aawsome]

To solve https://github.com/rustic-rs/rustic/issues/493 it will be necessary to add support for multiple parents.

This PR adds this support:

• The parent option can be now given multiple times
• In the SnapshotFile also mutliple parent ids can now be saved
• All trees of all parent snapshots are processed and a suitable entry of any of the parents is taken

[codecov[bot]]

Codecov Report

:x: Patch coverage is 64.78873% with 25 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 43.8%. Comparing base (2b8c4fd) to head (242beeb).

Files with missing lines	Patch %	Lines
crates/core/src/commands/backup.rs	57.5%	14 Missing :warning:
crates/core/src/archiver/parent.rs	71.4%	10 Missing :warning:
crates/core/src/repofile/snapshotfile.rs	66.6%	1 Missing :warning:

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/tests/integration.rs	81.4% <ø> (-4.3%)	:arrow_down:
crates/core/src/repofile/snapshotfile.rs	56.6% <66.6%> (-0.4%)	:arrow_down:
crates/core/src/archiver/parent.rs	63.5% <71.4%> (-1.8%)	:arrow_down:
crates/core/src/commands/backup.rs	54.3% <57.5%> (+6.8%)	:arrow_up:

... and 20 files with indirect coverage changes

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[simonsan]

Didn't thoroughly review, only read over it, but test seems to pass and in general looks good. 🖖