http: set default timeout in agent keepSocketAlive

Previous location of setting the timeout would override
behaviour of custom HttpAgents' keepSocketAlive. Moving
it into the default keepSocketAlive allows it to
interoperate with custom agents.

Fixes: https://github.com/nodejs/node/issues/33111

PR-URL: https://github.com/nodejs/node/pull/33127


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>