Cannot join two imported values

[indented-automation]

Description

Add-FnJoin fails when an Import value is used.

Support for Fn:ImportValue is described here:

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-join.html

Steps to reproduce

Add-FnJoin -Delimiter ',' -ListOfValues @(
    Add-FnImportValue -ValueToImport 'someValue1'
    Add-FnImportValue -ValueToImport 'someValue2'
)

Expected behaviour

Creation of a Join function call encapsulating the two import function calls.

Actual behaviour

Add-FnJoin: This parameter only accepts the following types: Vaporshell.Condition.If, Vaporshell.Function.Base64, Vaporshell.Function.FindInMap, Vaporshell.Function.GetAtt, Vaporshell.Function.GetAZs, Vaporshell.Function.Join, Vaporshell.Function.Select, Vaporshell.Function.Split, Vaporshell.Function.Ref, System.String. The current types of the value are: Vaporshell.Function.ImportValue, Vaporshell.Function, System.Management.Automation.PSCustomObject, System.Object.