Extension URL not validating properly

[LTA-Thinking]

Describe the bug
When validating a resource with an extension the extension's url property is not being properly validated. According to the FHIR spec: "Except for child extensions defined within complex extensions, the URL SHALL be an absolute URL."

To Reproduce
Steps to reproduce the behavior:

1. Validate a resource with an Extension element:

{
	"resourceType": "Patient",
	"extension": [
		{
			"url": "invalid"
			"valueString": "Invalid URL"
		},
		{
			"url": "https://fake.notreal/extension",
			"valueString": "Valid URL"
		},
		{
			"url": "https://fake.notreal/nested-extension",
			"extension": [
				{
					"url": "valid-nested",
					"valueString": "Valid nested URL"
				}
			]
		}
	]
}

Expected behavior
Top level extension URLs are validated to ensure they are absolute URLs, while sub-extension's URLs are not.

Version used:

• FHIR Version: All
• Version: 4.3.0

[ewoutkramer]

We're currently only validating rules that have been expressed explicitly in the StructureDefinition - so either structurally or using a FhirPath statement. The spec is indeed full of "textual" rules that would require manual coding, which we have not done yet. That said, it's on our medium-term planning to fix this, so I'll tag this as a validation enhancement.

[mmsmits]

We can validate this in both the instance that uses an extension, as in the StructureDefinition that defines an extension.
Let's fix it in the StructureDefinition, the author needs to be warned.
Please review the rules for extension.url before fixing this.