Make Evidence Immutable

[ben51degrees]

Background

The Evidence contained in FlowData is no completely immutable, though it is recommended in the specs.

The SequenceElement adds evidence during processing. It should instead use its own ElementData.

Proposed Changes

1. Make the Evidence immutable once the Process method is called,
2. Change SequenceElement to write to an ElementData instead of the Evidence,
3. Add a helper method which fetches the sequence from either the output of SequenceElement or the evidence,
4. Change any dependencies on SequenceElement results to use the helper, not the evidence.

For recommended implementation, see https://github.com/51Degrees/specifications/blob/main/pipeline-specification/features/evidence.md#adding-evidence-values