sending alert object as create alert response body

[sudhabindu-apm]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
This PR adds the sends the created alert as response body in create alert generic API.

Which issue(s) this PR fixes:
Fixes #3123

Screenshots:

Describe any introduced API changes:
This PR changes the response of the createAlert API (POST /api/v2/generic/incoming) from 204 to 200 with the alert object as the payload.

[mastercactapus]

For the response value, let's start as small as possible, and re-use existing field names from what already exists so we're consistent.

The closest we have for this is probably the webhook sender, so maybe something like this:

type CreateAlertResponse struct {
	AlertID     int64
	ServiceID   string
	IsDuplicate bool
}

reference: https://github.com/target/goalert/blob/master/notification/webhook/sender.go#L19

[sudhabindu-apm]

@mastercactapus I have made the requested changes.

[sudhabindu-apm]

@mastercactapus I have made the required changes. I have changed the IsDuplicate parameter of the returned object to IsNew as CreateOrUpdate is returning that so that we don't do unnecessary negation.

[allending313]

lgtm!

[sudhabindu-apm]

Thank you!