config: Refactor parsing of secrets section

[V02460]

Code refactor of the config’s secrets section. Can be reviewed commit-by-commit.

Benefits

• Makes some invalid states unrepresentable
• Eliminates now-unnecessary checks in the validate function
• Adds concurrent loading of key files
• Creates a cleaner separation between deserialization and creation of key store

Noteworthy changes

• This PR unifies the loading of keys from the config and file by no longer assuming a specific key format in either case.
• The futures crate is added as a dependency to have access to the standard async tools try_join and try_join_all.

[sandhose]

This is really cool, thank you!
Nice to see some concurrency in the loading, even though this is probably quite a insignificant optimisation :D

[sandhose]

Thanks again for doing this!