Allow get_parameter usage on TemplateRef steps/tasks

[elliotgunton]

Is your feature request related to a problem? Please describe.
When using a template ref in a DAG/Steps, you cannot access its outputs via get_parameter, because Hera tries to confirm the parameter exists, which is not currently possible for TemplateRefs.

Describe the solution you'd like
Allow TemplateRefs to use get_parameter syntax without validation (for now).

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Using the Argo templating syntax - {{tasks.prev-step.outputs.parameters.param-name}}

Additional context
Add any other context or screenshots about the feature request here.