Enrich "options.validation" for Auth0 connection.

[hypnoglow]

As Auth0 documentation states, there is a "options.validation" (defined
as an object), that can be applied to a Connection. Sadly, there are no specific documented
properties for that.

The use case and the problem

I want to set username min and max length for Auth0 database
connection - that options are available to configure through Auth0 UI.

To configure them through the Management API, I need to send the following json structure:

...
"options": {
    "validation": {
        "username": {
            "min": 2,
            "max": 10,
        }
    }

This works, and I also receive the same structure in the connection object
from the API for the "GET connection" request.

So, connection definition in terraform should look like this:

resource "auth0_connection" "default" {
  name     = "My-Database"
  strategy = "auth0"

  options = {
    requires_username = true

    validation = {
      username = {
        min = 5
        max = 15
      }
    }
  }
}

Currently, this does not work, because in the provider "validation" schema is defined as the following:

"validation": {
    Type:     schema.TypeMap,
    Elem:     &schema.Schema{Type: schema.TypeString},
    Optional: true,
},

So terraform wouldn't accept the auth0_connection definition above.

Another bad thing: if create a connection from terraform, then manually set
those validation properties on that connection through the UI, and then apply
the terraform again - they'll reset to default values.

BTW, I think this is related to #40 (same kind of issue)

[hypnoglow]

I made it work for the provider using the following code: #43

But that thing looks ugly, especially that mess with []map[string]interface{}-s and similar. I wonder if there is a better way to support validation properties.

[mattfysh]

also running into something similar, trying to set options.client_id and options.client_secret on a github connection

[github-actions]

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