network: introduce ManageForeignRoutingPolicyRules= boolean setting in networkd.conf

The commit 0b81225e makes that networkd
remove all foreign rules except those with "proto kernel".

But, in some situation, people may want to manage routing policy rules
with other tools, e.g. 'ip' command. To support such the situation,
this introduce ManageForeignRoutingPolicyRules= boolean setting.

Closes #19106.