Implement a type flag for `ec init policies`

[mbestavros]

As is, ec init policy emits a basic policy, geared towards ec validate image. That said, it might be possible that users will want to try writing their own policies for e.g. ec validate definition.

ec init policies should have an option to allow users to specify what kind of policy they'd like to generate - perhaps something like --type. It could have specific options, or it could take a string.

A/C:

• ec init policies has an option to control the policy type being created.