chore: Map external_source response to the nested output proto field

[Shreyansh228]

Within the context of the dagger post-processor, when enhancing data through an external API, the response is presently linked to an extra field within the output-proto. However, if the intention is to associate it with an existing nested field instead, the current approach falls short. This adjustment aims to enable the mapping of this response to the pre-existing nested field within the output proto. By doing so, the need for introducing the additional field will be avoided, resulting in a more efficient proto schema mapping for the output proto.

e.g
Mapping of external API response to the nested existing field driver-pickup-location.address in the output proto here
By this way we can avoid introducing new field address in the output proto.

"output_mapping": {
    "driver-pickup-location.address": {
	    "path": "$.driver_pickup_location.address"
    }
}