Make UUID implementation available for downstream projects

[bodduv]

What's Changed

This PR moves the UUID implementation from tests to appropriate locations so downstream projects can use them. This allows to avoid downstream projects to have their own custom implementation for a well-understood data type. A few modifications, such as the use ArrowBuf, are proposed.

Closes #823 .

[lidavidm]

The UUID type here was never meant to be a real type, it was implemented solely for testing things and doesn't match the UUID canonical extension type. If there's a desire for a UUID type, please open a new issue and implement the canonical extension type instead.

[bodduv]

@lidavidm thank you for the comment! I can open an issue and work on this. I have a couple of questions:

• would the canonical extension type implementation for UUID be considerably different the current PR proposes? Are there any concerns regarding the current implementation using ExtensionType, ExtensionHolder and ExtensionVector (apart from the fact that the implementation is moved and modified)?
• in case we add the canonical extension type for UUID, should we keep UuidType and other relevant code in tests?
  (tagging @wgtmac too)

[lidavidm]

It's possible they are the same, but at the very least the extension name here is wrong, and I would prefer it be clearly documented and in its own namespace, and for everything to be reviewed with that in mind instead of just moving test code into a feature that has to be supported long-term.