Unify `target` and `platform` parameters

[javiermtorres]

The YAML config and the python API support the target property (for example in

encoderfile/encoderfile-py/python/encoderfile/_core.pyi

Line 98 in df6455f

target: Optional[str | TargetSpec] = None,

), with the same semantics as the platform parameter in the CLI (see

encoderfile/encoderfile/src/builder/cli/build.rs

Line 28 in e266100

pub platform: Option<TargetSpec>,

). A single term should be used for this concept all across the codebase.