Add support for handling in-app variables

[mountaindude]

Add these commands:

variable-get

Options:
--app-id : App(s) whose variable(s) should be deleted
--app-tag : App(s) whose variable(s) should be deleted
--id-type: Type of identifier passed in the --variable option. ID or name.
--variable : Names or IDs of variables to extract
--output-format : JSON or table output

variable-delete

Options:
--app-id : App(s) whose variable(s) should be deleted
--app-tag : App(s) whose variable(s) should be deleted
--id-type: Type of identifier passed in the --variable option. ID or name.
--variable : Names or IDs of variables to extract
--delete-all : Delete all variables in one or more apps
--dry-run: Do a dry run

[mountaindude]

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