feat(config): Add hooks

[aawsome]

This PR adds run-before, run-after, run-failure and run-finally hooks for:

• all commands in the [global.hooks] config profile section
• commands accessing the repository in the [repository.hooks] config profile section
• the backup command specifically in the [backup.hooks] config profile section
• specific backup sources in the [backup.snapshots.hooks] section

Note: This PR includes only calling the given commands. If there is the wish for supplying information to the commands (env variables or parameter substitution), this should be covered by a separate feature request/PR.

closes #902

[nardoor]

I feel like this feature deserves:

• an integration test in some way
• a page in the cargo book docs

[aawsome]

@nardoor A use case could be:

[repository]
repository = "/mnt/repo/

[repository.hooks]
run-before = ["mount /mnt"]
run-finally = ["umount /mnt"]

which would simply mount and umount a (maybe external) drive before and after each rustic operation. Now, when copying to such a repository, we would like to run the hooks too.

(updated to repository.hook; after this is introduced, it is the better place to use such hooks)

[aawsome]

I rebased this to main (merging main into it was getting too complicated)..

I'd like to add [repository.hooks] as suggested - I think I can do it this evening. Then this PR will be ready, from my side.

[simonsan]

First iteration for text grammar in docs. Will review again, when the other changes have been pushed.

[simonsan]

Small nits regarding docs

[simonsan]

LGTM!