fix: remove global Client property on generated structs

[jedevc]

See discussion in discord: https://discord.com/channels/707636530424053791/1120503349599543376/1209815200023191662

#6680 made Client and Query reserved fields on all objects - but it's quite common to define Client on objects.

This patch is a little bit of a refactoring I've wanted to do for a while - it essentially allows binding clients to querybuilder.Selections, which means we can pretty much do away with Client+Query combinations, now we only need to track Query (and the client gets automagically inherited).

We still have this issue for Query. Gonna keep thinking about this one.

[jedevc]

@vito also suggested we could use XXX_ prefixing, as in

dagger/sdk/go/querybuilder/marshal.go

Lines 17 to 23 in f429f6e

	type GraphQLMarshaller interface {
	// XXX_GraphQLType is an internal function. It returns the native GraphQL type name
	XXX_GraphQLType() string
	// XXX_GraphQLIDType is an internal function. It returns the native GraphQL type name for the ID of this object
	XXX_GraphQLIDType() string
	// XXX_GraphqlID is an internal function. It returns the underlying type ID
	XXX_GraphQLID(ctx context.Context) (string, error)

[vito]

LGTM as-is, we can consider that XXX_ prefix in the future if/when this comes up again for Query (I suspect it will)