Add ability to pass encryption keys as strings

[jeqo]

(moving a comment on RSMConfig to this issue)

Currently, keys are passed as paths to key files. Check if there's demand to pass them as string instead.

[ivanyu]

BTW, some old WIP, may be somewhat useful: https://github.com/Aiven-Open/tiered-storage-for-apache-kafka/tree/ivanyu/keys-as-string