Add `reference` and `referenceId` fields instead of `productTemplateId` in milestoneTemplates

[vikasrohit]

To decouple the timeline/milestones from projects/phase/products and to be better at supporting timelines/milestones for other entities than project/phase/product, we need to use reference and referenceId in stead of productTemplatedId in ProductMilestoneTemplate model.
Purpose of this ticket to:

1. Rename the ProductMilestoneTemplate model to MilestoneTemplate
2. do the required changes in the model and its CRUD operations.
3. Create new endpoint /v4/milestoneTemplates which lists all the milestone templates for the given reference and referenceId
4. Update the /v4/productTemplates/123/milestones endpoint to fetch milestone templates using API call to the new endpoint created in 3.
5. Update the other places in project service itself, where we are looking for milestone templates based on productTemplateId e.g. project's create endpoint uses productTemplateId to look for all required milestone templates.
6. Update unit tests
7. Update swagger and postman docs

fyi @RishiRajSahu

[vikasrohit]

@coderReview this one is not critical for our upcoming release, however, we can get it done in current bug bash, after we have closed all issues for the current release.

[coderReview]

ok

[coderReview]

@vikasrohit all issues have been merged, however can you explain more the issue. There is no milestoneTemplate model.

[vikasrohit]

@coderReview I have put more details in the requirements. And I think we still need to put more details, so please wait on this one. We can group this in our next bug bash or private task challenge.

[ngoctay]

@vikasrohit

1. What are values of reference? project and product, right? (We don't have phase templates, so no need to support phase)
2. For /clone endpoint, currently we use sourceTemplateId to identify the source product template id. I think we will need to change it to sourceReference and sourceReferenceId, right?

[vikasrohit]

@ngoctay We need to support productTemplate as reference and id of the actual productTemplate as referenceId. We don't associate milestone templates with actual project or product.

For /clone endpoint your understanding is correct.

[vikasrohit]

Working fine. Closing.

@gondzo please feel free to tag it as per convenience for payments.