Adds server settings models

[desertaxle]

This PR adds four new settings models:

• ServerSettings
• ServerAPISettings
• ServerDatabaseSettings
• ServerEphemeralSettings

In general, many settings previously in the PREFECT_API_ namespace will move under the ServerSetting model.

Support for the old names is maintained via validation aliases for backward compatibility. A new source EnvFilterSettingsSource is also added to be able to filer environment variables. It turns out all entries in validation_alias are used to load values via environment variables. I ran into this with the user field on ServerDatabaseSettings model since Unix-based operating systems have a USER environment variable that was getting picked up. Now, all AliasPath alias choices will be excluded when looking for values via environment variables.

[codspeed-hq]

CodSpeed Performance Report

Merging #15758 will not alter performance

_{Comparing create-server-settings-models (0b25738) with main (dcf250b)}

Summary

✅ 3 untouched benchmarks

[zzstoatzz]

lgtm! just something I thought while reviewing, not necessarily an ask for this PR, do you think its worth adding gt / le for numbers where appropriate?

[desertaxle]

do you think its worth adding gt / le for numbers where appropriate?

Yeah, that's a good idea! We can add those at some point.