feat: add global startup method configuration for snapshot and mithril

[lowhung]

Description

Introduces a [global] configuration section that gets merged as a base layer for every module's config. This allows shared settings to be defined once rather than duplicated across multiple [module.*] sections.

Module-specific values take precedence over global values when keys collide. This requires the Caryatid PR to be merged for the new versions to be picked up.

[global.startup]
method = "snapshot"

[module.mithril-snapshot-fetcher]
# can access startup.method from global

[module.snapshot-bootstrapper]
# can also access startup.method from global

Both modules can now call config.get("startup.method") to decide whether to proceed or exit early—without duplicating the setting in each module's section.

Related Issue(s)

#220 & #395

How was this tested?

• Running the node using both "mithril" and "snapshot" as the startup.method

Checklist

• My code builds and passes local tests
• I added/updated tests for my changes, where applicable
• I updated documentation (if applicable)
• CI is green for this PR

Impact / Side effects

• No breaking changes: Existing module configs are unaffected

[SupernaviX]

I think it would make sense for a helper function which did this to live in common, specifically so that we automatically get the same default value everywhere. But I don't think that's critical enough to block the PR

[lowhung]

I like that, something like this perhaps?

use config::Config;

pub const CONFIG_KEY_STARTUP_METHOD: &str = "startup.method";

impl StartupMethod {
    pub fn from_config(config: &Config) -> Self {
        config
            .get::<StartupMethod>(CONFIG_KEY_STARTUP_METHOD)
            .unwrap_or(StartupMethod::Mithril)
    }
}

[lowhung]

Done here ede0b1c

[whankinsiv]

My only feedback is to move the key definition to common and also include the default value. const CONFIG_KEY_START_UP_METHOD: (&str, &str) = ("startup.method", StartupMethod::Mithril);