Add fetching of FABRID policy descriptions to the SCION CLI.

[jeltevanbommel]

This PR adds the option to fetch policy descriptions to the scion CLI tool. Largely based on jeltevanbommel#5

Concretely the CLI dials the local CS using the local SCION daemon. Then

• for a requested FABRID Local policy: the local CS checks whether the policy is already known locally (either because it is a policy it owns, or it has recently requested the same identifier and it is cached) and returns the description. If it is not known to the local CS, the local CS dials the remote AS that has announced the FABRID Local policy identifier and requests the description.
• for a requested FABRID global policy: the local SCION Daemon uses the global store of FABRID policies, which is configurable in the SCION Daemon configuration file, to request the policy description.

Other features:

• Pretty printing the result
• Add a special case for when we request a remote policy and it is a policy from our own AS.

Usage:

bin/scion fabrid G1100 --sciond $(./scion.sh sciond-addr 112)
bin/scion fabrid L1121 1-ff00:0:112 --sciond $(./scion.sh sciond-addr 112) --format human
bin/scion fabrid L1121 1-ff00:0:112 --sciond $(./scion.sh sciond-addr 112) --format json

Depends on #173

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