Block template sharing

Hi all,

I did a write up with a proof of concept of the idea of sharing block templates with peers, particularly in order to improve compact block relay when miners and nodes have differing mempool policies. It's at:

https://delvingbitcoin.org/t/sharing-block-templates/1906

I've now also written up a draft BIP specifying the p2p messages that uses, which can be found here:

https://github.com/ajtowns/bips/blob/202508-sendtemplate/bip-ajtowns-sendtemplate.md

Review welcome! If you prefer to make comments via a github review rather than here or on delving, I've opened a PR on my own repo at https://github.com/ajtowns/bips/pull/7 which you could use.

Cheers, aj