Option to add an arbitrary (non-member) email to alerts

### Problem Statement

I want to be able to send an alert to a group email address, without having to manage that group (as a team) inside of Sentry. Currently, when I add an action to "Send Email" I have to choose from existing members or teams.

If I have a team of people with emails a@example.com, b@example.com, c@example.com, and I also have a distribution group email that is team@example.com, I want to be able to register team@example.com to receive emails, instead of adding those members individually. That would mean that I don't have to manage both my DG internally and my Sentry team at the same time, and also that if d@example.com joined the team, they could get alerts without/before creating a user for them in Sentry.

### Solution Brainstorm

When talking about this, we said that we could create a new sentry account that uses the distribution group's email address, but since we use an SSO provider to login to Sentry, I don't really want to try to get around that.

It was also suggested that this might be a security risk, but I think I'm willing to take that risk if I can lock down the ability to create/edit alerts.

### Product Area

Alerts

No files, tests, or entry points are named. Start by locating the Alerts "Send Email" action and its existing member/team recipient flow, then define how arbitrary recipients and the stated security restrictions should work; done means a distribution-group address can receive alerts without a Sentry account.

Written by the indexing model from the issue text.