Allow base job template variables' properties to be valid placeholders

[kevingrismore]

Imagine I have a base job template with a variable that's an object with many properties. I want this variable to be a singular structured object for validation, readability, and conceptual relatedness. However, I also want to retain the readability and flexibility of the job configuration because of the way it can map directly to the shape of configuration that's submitted to infrastructure.

One change that could make this possible is accepting placeholders that use dot operators to represent paths to individual properties of variables. Right now, this fails validation upon attempting to update a work pool because the placeholders aren't referencing root level keys from variables:

{
  "variables": {
    "type": "object",
    "properties": {
      "name": {
        "type": "object",
        "title": "Full Name",
        "description": "A person's first and last name.",
        "properties": { 
          "first": {
            "type": "string",
            "title": "First Name"
          },
         "last": { 
           "type": "string", 
           "title": "Last Name" 
         }
      }
    }
  }
  "job_configuration": {
    "first_name": "{{ name.first }}",
    "last_name": "{{ name.last }}"
  }
}

The worker uses apply_values to fill in the template with variable references, which I'm pretty sure would enable this to work if it were allowed.

from prefect.utilities.templating import apply_values

template = """
Hello, {{ name.first }} {{ name.last }}!
"""

applied = apply_values(template, {"name": {"first": "John", "last": "Doe"}})
print(applied)

>>> Hello, John Doe!

I'm curious if anything about this pattern complicates the work pool templating concept too much.