Implement Annotation config types

[dmartmillan]

Description
Port openvariant/annotation/config_annotation.py enums to Rust serde-compatible types: AnnotationType, AnnotationDelimiter, AnnotationFormat.

Files

• src/annotation/config.rs — NEW
• src/annotation/mod.rs — NEW

Definition of Done

• AnnotationType covers Static | Internal | Dirname | Filename | Mapping | Plugin
• All variants derive Serialize, Deserialize, Debug, Clone, PartialEq
• serde(rename_all = "lowercase") matches YAML keys exactly

Documentation needed: No external docs needed; add rustdoc comments on each variant.

Tests needed: Rust unit test in the same file — round-trip YAML parse → serialize for each variant.

Depends on: #61

Note: perform an accurate validation of the YAML annotation format. Detect all possible syntax, structural, and semantic errors, and provide clear explanations to help users quickly identify any issues.

[FedericaBrando]

Is this going to validate the config as well? I think one very hard thing to debug in openvariant is when the yaml is misconfigured. We should give clean errors to the users.