Related: #6100

This PR extends the rich parameters feature to let users mark them as optional. This behavior is consistent with Terraform variables.

A rich parameter is considered required when it doesn't contain the default property, so:

data "coder_parameter" "another_required_field" {
  name        = "Another required field"
  icon        = "/icon/aws.png"
  description = "This string is required"
  mutable     = true
}

If we want to mark the parameter as optional, we need to add the default property:

data "coder_parameter" "random_string" {
  name         = "Optional random string"
  icon         = "/icon/aws.png"
  description  = "This string is optional"
  mutable      = true
  default      = ""
}

Changes:

• use the latest terraform-provider-coder to support computed property "optional" (not exposed to users)
• adjust coderd API to expose required property
• site: allow for empty, optional parameters
• site: use HTML API to check if field is required
• modify CLI to allow for entering values for newly added parameters to templates

Follow-ups:

• present a special form or popup to enter required params without defaults (especially if those are new parameters)