feat: API to delete given social auth record for user

[kaustavb12]

Description

This PR introduces an API to delete a social auth record with provided UId for a given user.
This API is intended for server to server calls from third party system.

Testing instructions

1. Checkout and setup this PR branch
2. Go to social auth admin page https://<LMS_HOST>/admin/social_django/usersocialauth/ and create a new social auth record for any user.
3. Invoke DELETE call to url https://<LMS_HOST>/api/third_party_auth/v0/providers/modify_auth with JWT access token from a staff/admin user. Here's a sample JSON request body:

{
    "username_or_email": "edx",
    "uid": "abcd1234"
}

4. Verify the auth record with given UID is deleted.

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

Other information

Private Ref: BB-9477

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[xitij2000]

Perhaps this could be added as a delete verb for the existing UserViewV2?

[kaustavb12]

@xitij2000 You are right. I have modified the PR to now add delete verb to existing UserViewV2.
Please take a look.

[xitij2000]

I think the code generally looks good and is working fine, but there are a few changes I'd suggest.

1. We don't need to return anything from delete.
2. We can use single query instead of three here.

[xitij2000]

This is an odd place to import the User model from. Please use get_user_model instead from django.contrib.auth import get_user_model

[xitij2000]

I think it's better to condense this into a single query:

UserSocialAuth.objects.get(**{"user__"+identifier.kind: identifier.value}, uid=uid).delete()

Conventionally, delete doesn't return any data, so you can switch to a 204 status code and return an empty response.