rpk: allow names to be URIs in the reference flag when creating a schema.

[r-vasquez]

jsonschema will introduce URIs as valid reference names.

Backports Required

• [ ] none - not a bug fix
• [ ] none - this is a backport
• [X] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.1.x
• [ ] v23.3.x
• [ ] v23.2.x

Release Notes

• none