[JSON Logic] Part 2: Computed Attribute skeleton

[brennj]

Requires: https://github.com/remoteoss/json-schema-form/pull/34 and https://github.com/remoteoss/json-schema-form/pull/35 for full context and to work correctly. Must only be merged when they are.

This is a breakdown of https://github.com/remoteoss/json-schema-form/pull/28 to make it more digestible for review.

Add in x-jsf-logic-computedAttrs (with reference to values defined in x-jsf-logic.computedValues) support.

• Ensures when there is a x-jsf-logicComputedAttrs are present, it reads from x-jsf-logic.computedValues for that computed value.
• From this PR, any plain string based single value is supported. (Where you need titles, etc, inline rules, they will come in following PRs)
• Forced values are supported for this.
• Adds a test to show how this works for basic values (const, default)