Add stacktrace.function to alerting condition filters

### Problem Statement

Currently, Alerts have the following stacktrace options:
stacktrace.code
Stacktrace.module
stacktrace.package
stacktrace.filename
and etc.

What the user needs in alerts conditions is stacktrace.function.
Example: I can search for: "stack.function:*customviews::SentryView*", and they would like to create an alert based on that search.

The current workaround is to create alerts based on stacktrace.package xyz, which produces too many unrelated alerts that are not useful. Stacktrace.function alerts (once added) can then be directed to specific team and specific developers that are responsible for that part of the code if that function produces crashes.

### Solution Brainstorm

Make stacktrace.function available as an event attribute that you can filter on when configuring alerts

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-1805) by [Unito](https://www.unito.io)