ges-structure-parser: force string types

ges-structure-parser: force string types

Force a string type for structure values obtained through parsing a
serialized timeline by inserting a (string) specifier after a '=',
rather than relying on gst_structure_from_string guessing the type.

As such, the functions that extract clocktimes and properties are
modified to accept string value types.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/123>