Release: Automatic diff snippet inclusion in release notes

[bkchr]

When generating the release notes from the prdoc files, it would be nice to include certain changes as a diff in the release notes as well. For example when a pallet Config gets changed, the diff should show the changes that are required for the implementation of the trait. This diff should make it easier for downstream users to replicate the changes.

To capture these diffs, we should create a dedicated crate that represents a runtime that includes all the pallets available in polkadot-sdk. We could maybe repurpose kitchensink-runtime for this, but it currently misses the relay chain pallets. For the node changes we could probably capture the diffs from one of the templates. These crates also probably need to have special sections (highlighted by special comments) in which the diff should be captured. So, that changes outside of these sections to not appear in the diff.

A possible future step could be a tool that takes all these diffs and applies them automatically when doing an upgrade of the polkadot-sdk version. It could ask for input when a new value is introduced or just write todo!() with some proper comment. Basically to make it much easier to upgrade to a new polkadot-sdk version.

CC @ggwpez @EgorPopelyaev @Morganamilo

[ggwpez]

It would be very nice to have this, but I am not sure if we have the resources to do this. It seems to me that there are only a few things that we can really improve now before the release team gets new people.
I could set up a survey for parachain teams to find out what the most annoying part about Releases is for them