Create a `constellation_traits` crate (#36088)

This change creates a `constellation_traits` crate. Previously messages
to the `Constellation` were in the `compositing_traits` crate, which
came about organically. This change moves these to a new crate which
also contains data types that are used in both compositing/libservo and
script (ie types that cross the process boundary). The idea is similar
to `embedding_traits`, but this is meant for types not exposed to the
API.

This change allows deduplicating `UntrustedNodeAddress`, which
previously had two versions to avoid circular dependencies.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

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