GET/UPDATE/DELETE of subcondition

We want to update a single subcondition
hence try to find the path of the subcondition or better ways to navigate subconditions

example

```
GET/DEL/UPDATE fulfillment.subcondition_by_path([(type_id=cc.types.THRESHOLD, args=[branch_nb]), (type_id=cc.types.ED25519, args=[public_key]))]
```

the args list

```
[(type_id=cc.types.THRESHOLD, args=[branch_nb]), (type_id=cc.types.ED25519, args=[public_key])]
```

is an ordered selector for each depth in the tree so at depth=0 you will look for a `THRESHOLD` and select subcondition with `branch_nb`

Start by examining the existing subcondition navigation API and the proposed subcondition_by_path example, including the ordered type_id and args selectors at each tree depth. Define the supported GET, UPDATE, and DELETE behavior and confirm that a single nested subcondition can be selected unambiguously before implementing and testing it.

Written by the indexing model from the issue text.