Allow to encrypt specific nodes in a file with specific keys (muliple matching creation_rules)

it would be really awesome to be able to encrypt parts/specific nodes with different keys. as far as i have seen it is currently only possible to encrypt a whole file with a specific set of keys.

sometimes it would be really handy to be able to encrypt only specific nodes/keys value pairs with a specific set of keys.

maybe the creation_rules can do that. having a .sops.yaml file with multiple creation_rules will currently only match the first match of path_regex but it maybe it can continue (for example with a flag set in the creation_rule like continue or the oposite stop_propagation) to the next rule so we could have something like this:

creation_rules:
  - encrypted_regex: 'docker_registry_password'
    path_regex: .*/dev/.*\.yaml$
    age: 'A'
    continue: true
  - encrypted_regex: '.' # prevent encryption of comments https://github.com/mozilla/sops/issues/921
    path_regex: .*/dev/.*\.yaml$
    age: 'A,B'
    propagation_stop: true

#   catch-all
  - encrypted_regex: '.' # prevent encryption of comments https://github.com/mozilla/sops/issues/921
    age: 'C'

for yaml files which are in the dev folder the docker_registry_password will be encrypted with key A. if no continue would be present, it would be the only node which is encrypted and the rest is unencrypted. having the continue in place leads the processing the next rule which also matches the path_regex. all other nodes are now encrypted with key A and B. with propergation_stop the processing stops after matching this rule.

this are just thoughts, i don't know if it is possible from the current code design. still it would be really awesome.

related to: #1196 #1392

The proposal concerns .sops.yaml creation_rules and matching YAML nodes, with related discussion in issues #1196 and #1392. Start by tracing how creation_rules are selected, then determine whether multiple matching rules can be composed safely. Done means specific nodes can use distinct keys while continue and propagation_stop produce predictable results, with coverage for overlapping rules.