[REQ] Allow authors of custom templates to apply multiple templates to api/model arrays

[jimschubert]

Is your feature request related to a problem? Please describe.

Currently, users writing custom templates can only apply the templates supported by the built-in generator. If a generator doesn't support something like service interfaces or model test classes, the user can't specify these in their custom templates.

Describe the solution you'd like

Ideally, an extension to a configuration file to allow extending for a specific generator:

{
…
"modelTemplate": {
    "merge": false,
    "files": [
        { "template": "model.mustache", "extension": ".m"},
        { "template": "model_license.mustache", "extension": ".license"}
    ],
"apiTemplate": {
    "merge": false,
    "files": [
        { "template": "api.mustache", "extension": ".a"},
        { "template": "api_interface.mustache", "extension": ".ai"},
        { "template": "api_license.mustache", "extension": ".license"}
    ]
}
…
}

This doesn't address some things like output structures which would be required by generators.

Describe alternatives you've considered

n/a

Additional context

see #690