[CORE-3182] Schema Registry json external references

[BenPope]

Currently just a rebase of #23677 with:

• Fixup for schema_getter
• Shuffling of code to later commits to make each commit compile (rather than error with unused).

Backports Required

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

Release Notes

• none

[andijcr]

I see no bugs in this pr