Add issue template for Audit Event Proposal

GitLab community members, including GitLab team members, frequently
request new AuditEvent types. This introduces an issue template to
facilitate and standardize those proposals.

.gitlab/issue_templates/Audit Event Proposal.md

+<!-- Audit Event documentation: See https://docs.gitlab.com/ee/administration/audit_events.html -->
+
+## Audit need
+
+<!-- Describe the real-world use case for the audit event you want to introduce, and explain the closest thing that GitLab already captures. -->
+
+## Proposal
+
+<!-- Describe the audit event you are proposing should be added, including any details of what should be captured, how, and why. -->
+
+/label ~"Category:Audit Events"
+/label ~"feature"
+/label ~"group::compliance"