lib/static-delta: document and check parameters format

lib/static-delta: document and check parameters format

This enhances the logic handling GVariant parameters within
`ostree_repo_static_delta_generate()`.
Several of those entries are expected to be zero-terminated values,
and this implicit assumption has been observed to be an hidden trap
in languages where strings and arrays may not carry a terminator value
(e.g. Rust).
In order to improve the situation, this makes the documentation more
explicit and actively tries to catch invalid input parameters.