Peek an extra row so hasNextPage avoids a second query

[drhops]

Stacked on #5773.

Problem

When a query selects both the page and pageInfo { hasNextPage }, RelationConnection runs two queries. One loads the page and the other checks for a row past it. The spec already notes this: "A better implementation would load first + 1 nodes and use that to set has_next_page."

Change

When hasNextPage is selected and the page is loaded anyway (nodes, edges, startCursor or endCursor), the connection loads first + 1 rows. It returns first of them and sets has_next_page from whether the extra row exists. That's one query instead of two.

This follows the lookahead approach suggested in #4908 :

• ConnectionExtension adds extras [:lookahead], and Connection gets a lookahead accessor.
• The lookahead is attached in ConnectionExtension#after_resolve and in Execution::Next's FieldResolveStep#finish_extensions, because Next doesn't call after_resolve. In both, the field's own arguments stay free of the lookahead.

What doesn't change

• If hasNextPage isn't selected, the query fetches exactly first rows.
• If only hasNextPage is selected, it keeps the existing separate check.
• last and before, a relation already limited below first, and connections built without a lookahead all behave as before.

Cost

Every connection field now builds a Lookahead when it resolves (a few small objects), and :lookahead shows up in field.extras.