Improve customizability using a local configuration file

[peintnermax]

To reduce implementation effort for self hosters, a local configuration file should be introduced which contains diverse options for:

• branding/appearance
• behaviour
• features

This will improve the customizability and centralize the settings for easy extension. Implementation details will follow.

[KieronWiltshire]

As much as this would help, it will never satisfy strict style guidelines some companies have. I think headless components are a better way to go.

[peintnermax]

Please check out our newest version in our main Zitadel repository. This guide explains enhanced customization options. let us know what you think! https://github.com/zitadel/zitadel/blob/main/apps/login/THEME_CUSTOMIZATION.md