Add new field `metadata` to the `milestoneTemplate` model

[vikasrohit]

This new field would be of type json which would serve as storing milestone's type specific configuration e.g. for final-designs milestone type, we have to store configuration for the number of final designs that have to be selected to move further.

Scope of this tasks is to:

1. add new field metadata to the model
2. accept this new model in create endpoint. it would save as field as is.
3. accept this new model in update endpoint. It would merge the incoming json with existing json as follows (Please see the implementation in projectTemplate update endpoint for reference):
   3.1 It should merge non array fields
   3.2 override the array fields with incoming value
   3.3 delete the null valued keys
4. Update/Add unit tests to test the new field and its update logic
5. Update postman and swagger docs

[vikasrohit]

Closing as working fine as per given requirements.

@gondzo Please feel free to tag it as per convenience for payment.