Support for drop-down or choice selection in additional-input section.

[shrajfr12]

additional-input section in metadata.yaml file in addons subdirectories takes on fields specified like

additional-input:

label: l1
key VAR
type: text
default : something
For some fields it might be necessary to provide a choice of several inputs with only one selectable.

For example, if the field is for supported version numbers 1, 2, 3, we might introduce a new keyword "choices": oneof "1", "2", "3" and conjure-up woud display the three values and allow one of them to be selected.

Current environment details
juju version = 2.3.3-xenial-amd64
conjure-up --version = conjure-up 2.5.5
Canonical-Kubernetes, AWS

This is a request for enhancement to improve user-experience

[stale]

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