turn secrets encryption into "per field" option instead of separate files

[ctheune]

[elikoga]

Will include in #154

[elikoga]

I think this is https://github.com/flyingcircusio/batou/pull/440 and solved

[zagy]

@elikoga doc/source/user/age.md should mention the new format.

[zagy]

After encrypting I cannot edit the file (2.5b1):

#  ./batou secrets edit staging
Trying to assign a multi-line value to <Option 'id_ed25519'>.
Use the ``set_values`` or ``append`` method to accomplish that.

[zagy]

This has been fixed.