Implement resource-management experimental feature

Adds basic resource tracking to system features used by distributed
builds, similar to resource management in job schedulers like Slurm.

Includes a positive and negative functional test and a documentation
update to the distributed builds section.

Resolves #2307

Signed-off-by: Lisanna Dettwyler <lisanna.dettwyler@gmail.com>

tests/functional/build-remote-resource-management-should-fail.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+enableFeatures "resource-management"
+
+requireSandboxSupport
+[[ $busybox =~ busybox ]] || skipTest "no busybox"
+
+here=$(readlink -f "$(dirname "${BASH_SOURCE[0]}")")
+export NIX_USER_CONF_FILES=$here/config/nix-with-resource-management.conf
+
+expectStderr 1 nix build -Lvf resource-management.nix \
+  --arg busybox "$busybox" \
+  --out-link "$TEST_ROOT/result-from-remote" \
+  --store "$TEST_ROOT/local" \
+  --builders "ssh-ng://localhost?system-features=testf - - 4 1 testf:1" \
+| grepQuiet "Failed to find a machine for remote build!"