Add policy serialisation primitives to mismi-iam

[olorin]

I added types/serialisation stuff for policy documents in moog, but it's generally applicable and should probably go in mismi-iam instead, unless there's a better place I'm not aware of.

[markhibberd]

mismi-iam is probably good @nhibberd wrote some in bucket with some quasi-quoters as well that should probably come in.

[nhibberd]

Yeah ideally will lift bucket code up

[olorin]

I just did plain Haskell types with aeson - don't have a strong preference, but it seems like the QQ implementation in bucket doesn't do a lot to ensure that the provided policy is well-formed (e.g., version string is actually a valid version, et cetera)?

[nhibberd]

How did you do the permissions?

[olorin]

@nhibberd currently "Action" is just a newtype around Text (like this); wasn't sure if it'd be worth enumerating everything at the type level - open to suggestions there.