Support of (optional?) size attribute for localBlob access

[zkdev]

What would you like to be added:
Add support for (optional?) size attribute for artefact-access of type localBlob.
Other artefact-access types already support size attribute, e.g. ociBlob.
Semantics should stay the same:

size integer: The size of the blob

Why is this needed:
Many specs, such as GNU Tar or OCI Content Descriptor, require size information when processing data.
When processing OCM artefacts via network, streaming is preferred as it prevents OOM-related errors, or expensive IO operations. When combining aforementioned technologies and streaming, it becomes clear that users need a way to know the size of an artefact, before actually transmitting it.

[fabianburth]

We had a quick discussion, as a preliminary work around, @zkdev will fetch the manifest of the component descriptor, search for the layer with the hash value of localReference of the respective resource and use the size attribute specified in the layer description.