Expose invariant IDs as an extension on IssueComponent

[andrzejskowronski]

Closes #615

This pull request introduces a consistent enhancement across the Firely FHIR Validator codebase by updating the way validation results are constructed. Specifically, it ensures that when an IssueAssertion is converted into a ResultReport, the originating assertion object is included as context. This change improves traceability and debugging of validation issues by allowing consumers to identify the exact assertion that led to each validation result, and to annotate the results with the information contained by the specific assertions.

The most important changes are:

Enhancements to Assertion Context Propagation:

• Updated all calls to AsResult in various validators (e.g., BindingValidator, CanonicalValidator, CardinalityValidator, ChildrenValidator, ExtensionContextValidator, FhirPathValidator, FhirStringValidator, FhirTxt1Validator, FhirTypeLabelValidator, FhirUriValidator, FixedValidator, InvariantValidator, ExtensionSchema) to pass the current assertion object (this or relevant instance) as an additional argument, ensuring the assertion context is preserved in validation results.

Core Assertion and ResultReport Infrastructure:

• Added an Assertion property to the IssueAssertion class and updated its constructor to accept and store an IAssertion instance, so that each assertion can be traced back from the result. [1] [2] [3]
• Modified the Validate method and AsResult overloads in IssueAssertion to ensure the assertion context is always included in the resulting ResultReport. [1] [2]

Minor Refactoring and Consistency Improvements:

• Introduced a constant CONTEXT_INVARIANT_KEY in ExtensionContextValidator to avoid magic strings and improve maintainability. [1] [2] [3]
• Cleaned up unused using statements in IssueAssertion.cs.

These changes collectively enhance the reliability and maintainability of the validation framework by providing better introspection into validation failures and their origins.