Support sbox-in-RBE

Allow passing a remoteexec.REParams to RuleBuilder to configure it to
run the rule remotely through RBE.  Requires the rule to use
SandboxInputs, which ensures that RuleBuilder is aware of all of the
inputs and outputs of the rule.

Running sbox in RBE initially seems unnecessary, as RBE is already a
good sandbox, but reproxy can execute RBE actions locally when configured
for local execution, local fallback or racing.  Using sbox in RBE ensures
that these local actions are also sandboxed, giving consistent results
between directly executed actions, local RBE actions, and remote RBE
actions.

Bug: 182612695
Test: manual
Change-Id: Icf2f24dde8dee833eb680ba22566a8e1c0143b15