overhaul cli flag implementation

[ChausseBenjamin]

The service now uses urfave/cli/v3 to parse cli flags. Every flag (for which it makes sense) can now be configure via either cli, environment variables, or a config toml file. Priority from most important to least important goes as follows:

1. direct cli flags
2. environment variables
3. config file
4. default values

Just like the existing comments in the codebase outlined, the --help message and the toml config is split into groups:

• PROXY protocol
• Prometheus metrics
• GeoIP
• Healthcheck
• Logging

Since environment variables is generally the more idiomatic way of configuring docker-compose infrastructure, the example yaml files now use that syntax. An example config.toml file has also been provided in that same examples/ directory.

The change also deprecates a few flags (marking them as such but still accepting them for backwards compatibility):

• interval_ms -> interval (which accept any Duration like s, ms, min, etc...)
• proxy_protocol_read_header_timeout_ms -> proxy_protocol_read_header_timeout (also accepts any Duration)
• prometheus_clean_unseen_seconds -> prometheus_clean_unseen (also accepts any Duration)
• enable_prometheus -> prometheus_enabled (was already deprecated but uses scaffolding provided by urfave/cli)

[ChausseBenjamin]

A few other things to note:

• ports now use uint16 (as ports are 16bits) since urfave/cli can parse those and that provides free bounds checking
• all cli strings are defined as constants so it's harder to make typos if you use those instead of strings to retrieve data from context