config: support checkout_jobs

[JohnAtl]

Bug Report

checkout: slow checkouts

Description

Checkout copies all files in parallel, leading to disk saturation, and excessive checkout times. E.g. At this time, lsof for the dvc process shows 331 files open.

Reproduce

dvc pull

Expected

Parallelization in moderation, respecting the jobs: parameter in .dvc/config, or some similar parameter.

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 3.11.1 (pip)
-------------------------
Platform: Python 3.10.10 on Linux-6.1.0-11-amd64-x86_64-with-glibc2.36
Subprojects:
	dvc_data = 2.10.1
	dvc_objects = 0.24.1
	dvc_render = 0.5.3
	dvc_task = 0.3.0
	scmrepo = 1.1.0
Supports:
	http (aiohttp = 3.8.5, aiohttp-retry = 2.8.3),
	https (aiohttp = 3.8.5, aiohttp-retry = 2.8.3),
	ssh (sshfs = 2023.7.0)
Config:
	Global: /home/john/.config/dvc
	System: /etc/xdg/dvc

Additional Information (if any):

https://discuss.dvc.org/t/is-jobs-n-ignored-on-local-stores/1768