Update assisted service to support external platform in agentClusterInstall

User Story:

As a user of the agent-based installer, I want to be able to:

• validate the external platform type in the agent cluster install by providing the external platform type in the install-config.yaml

so that I can achieve

• create agent artifacts ( ISO, PXE files)

Acceptance Criteria:

Description of criteria:

• install-config.yaml accepts the new platform type "external"
• agent-based installer validates the supported platforms
• agent ISO and PXE assets should be created successfully
• Required k8s API support is added

(optional) Out of Scope:

Detail about what is specifically not being delivered in the story

Engineering Details:

• (optional) https://github/com/link.to.enhancement/
• (optional) https://issues.redhat.com/link.to.spike
• Engineering detail 1
• Engineering detail 2

This requires/does not require a design proposal.
This requires/does not require a feature gate.