feat(config): allow to override some config values by hostname

[Altahrim]

Summary

Allows to set configuration based on hostname.
With this PR, we can have a unique server ID for Snowflake IDs, easily linked to the right server.

Variable priority

serverid (and other host defined variables) will be taken from:

• environment
• hostname configuration ($CONFIG_HOSTNAME)
• general configuration ($CONFIG)

TODO

• Allows to configure serverid with environment variable

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

[artonge]

Sorry, mostly comments about the snowflake part 🐵

[Altahrim]

Adding variables specific to server in the configuration seems risky for now.
If config.php is written from Nextcloud it may change the value for all hosts (because of the merge).
I'll open a new PR with only serverid and it's use with snowflake IDs