Adding syntax for the json config files to actually merge, not override, entries

I would like to see syntax for the json config files to actually merge, not override, entries.
Like to be able to just add a single import to the import list rather than overriding the whole list, or add a handler to the list of handlers via additional config .json.
Such an option would help a lot.

Describe the solution you'd like
I propose the following syntax:
Instead of having to override the whole list, e.g:
inference.json:

    "imports": [
        "$import numpy",                                                                            
    ],

inference_trt.json:

    "imports": [
        "$import numpy",
        "$from monai.networks import trt_compile"                                                                                                   
    ],

let's allow :
inference_trt.json:

   "+imports": [
       "$from monai.networks import trt_compile"                                                                                                   
   ],

Processing would be : let's add an extra merge pass after the 'override' step, where config entries are overridden by .update().
With '+' key prefix, both the original 'imports' and '+imports' entries will be in the dict.
Additional pass would find all the '+*' entries and perform merge of '+id" key entry into corresponding 'id' key.
Dictionaries will be update()'d, lists append()'ed, error otherwise.

Start by locating the JSON configuration override processing and any existing tests for configuration merging; the issue does not name specific files or entry points. Define done around the proposed +key syntax: dictionaries update, lists append, and unsupported value types report an error.