refactor(repository)!: Add more control over used keys

[aawsome]

Adds more control over the used repository keys. This is needed to implement the missing key list, key remove commands in the rustic CLI.

As this refactors the Repository a bit, this is a breaking change (which will most likely not harm anyone using rustic_core)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 55.31915% with 21 lines in your changes missing coverage. Please review.

Project coverage is 42.3%. Comparing base (213665e) to head (241bb84). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/repository.rs	56.0%	11 Missing :warning:
crates/core/src/backend/decrypt.rs	50.0%	6 Missing :warning:
crates/core/src/commands/check.rs	0.0%	1 Missing :warning:
crates/core/src/commands/prune.rs	0.0%	1 Missing :warning:
crates/core/src/repofile/keyfile.rs	66.6%	1 Missing :warning:
crates/core/src/repofile/snapshotfile.rs	50.0%	1 Missing :warning:

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/commands/init.rs	93.7% <100.0%> (ø)
crates/core/tests/integration.rs	84.6% <ø> (-0.6%)	:arrow_down:
crates/core/src/commands/check.rs	61.4% <0.0%> (+0.1%)	:arrow_up:
crates/core/src/commands/prune.rs	64.5% <0.0%> (-0.7%)	:arrow_down:
crates/core/src/repofile/keyfile.rs	72.5% <66.6%> (+5.4%)	:arrow_up:
crates/core/src/repofile/snapshotfile.rs	48.5% <50.0%> (-1.5%)	:arrow_down:
crates/core/src/backend/decrypt.rs	47.8% <50.0%> (-0.8%)	:arrow_down:
crates/core/src/repository.rs	43.3% <56.0%> (+0.1%)	:arrow_up:

... and 25 files with indirect coverage changes

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[aawsome]

rebased this PR to current master

[aawsome]

@simonsan is there anything open which prevents this PR from being merged? (I think we should anyway release a 0.8.0 version, so IMO the breaking changes are no blockers...)