Add a multiple choice field type for power parameters

[cpg1111]

Describe the bug
For the IPMI power driver, we're extending the parameters to allow the user to set workaround flags (ipmipower -W <flag>). This field can take multiple flags, and so the power parameter field type multiple_choice has been added on the backend, but the UI still needs to add an input for this type in the power parameters.

maas-ui version
HEAD (this is for the next release)

MAAS version
HEAD (this is for the next release)

LP issue
lp:1953395

To Reproduce
N/A

Expected behavior
Render a multiple choice field.

[Caleb-Ellis]

The changes we need to address include:

1. Rendering a multiselect if the field's field_type === "multiple_choice"
2. Handling a default value that could be an array of values