major(core|persisted): use GET for queries by default

[JoviDeCroock]

Summary

I've been thinking about this for quite a while but it's time that this default is switched up, most GraphQL clients force you in this paradigm of always leveraging POST which misses out on a lot of benefits. The settings to actually do it the "right way" HTTP semantically is hidden behind an option.

The downside is that some servers might not support GET, however they are still free to turn this off. I prefer the explicit having to turn it off from having a degraded default experience.

The default should still be persisted operations but at least we take a step forward HTTP semantically wise.

[kitten]

I think it's fair to push for this and change the default here.
This also enables us to look at what we can do to make this behaviour more expected in core.

Typically, query-level approaches omit the AST. We'll have to look at how we can deal with the lack of the AST, or rather, enforce that it's still there if a cache (or other logic) expects it.

Basically, we can be more explicit about:

• persistedExchange behaviour (potentially in core)
• how exchanges deal with a missing document (explicit incompatibilities)
• how exchanges deal with a document + hash (explicitly disallow modifications via freezes in development)