Add support for GitHub Audit Log Streaming

[derkkila-splunk]

Is your feature request related to a problem? Please describe.
With GitHub adding Audit log streaming as a service, please make sure that those events will work alongside audit log events collected via the GitHub Audit Log Monitoring Add-On for Splunk or via syslog forwarding from GitHub Enterprise Server.

Describe the solution you'd like
Streamed audit logs appear in the Audit dashboards

Describe alternatives you've considered
n/a

Additional context
n/a

[derkkila-splunk]

There is a slight delay between when the audit event is created and when it gets streamed. Created a new sourcetype github_audit that uses the @timestamp field for establishing the Splunk event timestamp. Otherwise, streamed logs work exactly the same as TA fetched ones.