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]

Codecov Report

Attention: Patch coverage is 57.44681% with 20 lines in your changes missing coverage. Please review.

Project coverage is 42.2%. Comparing base (6a50c12) to head (d456270).

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

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/commands/init.rs	93.7% <100.0%> (ø)
crates/core/src/commands/prune.rs	65.2% <100.0%> (-0.1%)	⬇️
crates/core/tests/integration.rs	80.7% <ø> (ø)
crates/core/src/commands/check.rs	61.8% <0.0%> (-0.4%)	⬇️
crates/core/src/repofile/keyfile.rs	65.1% <66.6%> (+0.5%)	⬆️
crates/core/src/repofile/snapshotfile.rs	49.7% <50.0%> (+1.4%)	⬆️
crates/core/src/backend/decrypt.rs	47.8% <50.0%> (-1.9%)	⬇️
crates/core/src/repository.rs	42.5% <56.0%> (+0.2%)	⬆️

... and 17 files with indirect coverage changes

[simonsan]

Two things:

• what happens in case two keys have the same starting strings?
• there is no safe-guarding in place, before deleting the keys (although that might fit better into rustic-rs

[aawsome]

find_id returns an error if no unique id can be found.
The safeguard is the check against key_id() - which is the key used to open the repo.