Add events for server startup completed

[dktapps]

Description

This was suggested years ago as part of a much larger proposal (#2828), which has since been rejected and broken down.

This would run at the end of server startup as seen in 38ce83d.

Justification

Honestly not too sure.

Alternative methods

This is already achievable by scheduling a delayed task with a delay of 0 during startup.
However, if there's a significant demand for it, an event might be a nicer way of doing this.

[dktapps]

Similar idea as StartupPostWorldsLoadedEvent as proposed in #6330, but in a way smaller proposal.