Store constraint information in node parameters

Store constraint type and parameters as node parameters.

Suggest parameter names:

- `MarkupConstraint_Kind` - the key for the constraint to use
- `MarkupConstraint_Params` - constraint dependencies

I suggest MarkupConstraint_Params should be a list of strings for indirection. Each element should be a tuple of strings with an adaptor ID used for deserialization.

I suggest to use UUID for idirection when dealing with node references or more complex parameters.

For example an entry in `MarkupConstraint_Params` of `('ControlPoint', '', '7')`
Would use the appropriate `ControlPoint` adaptor to refer to the control point with id `'7'` in node `node.GetNodeReference('')`. Note that this might be a self-reference.

The same adaptor should be responsible handling observations (see #4).

The issue names no files, tests, or entry points. Start by reviewing how nodes currently store and serialize parameters, then read the related observations issue #4 and assess the proposed MarkupConstraint_Kind, MarkupConstraint_Params, adaptor IDs, and UUID references. Done should include an agreed design and working storage and deserialization behavior for the proposed constraint data.

Written by the indexing model from the issue text.