Add support for custom entities

[a-asaad]

Motivation and Context

Custom entities provide an escape hatch for users to add custom XML snippets to the diagram output. These entities are not shown in the diagram, but can be referenced from a node's attributes.

Details

• Schemas: add a customEntities field to the EipFlow schema
• Flow Translators: accept customEntities from EipFlow translation requests, validate, and add to XML output.
• UI
  • Add a panel to allow creating and managing custom entities.
  • Add a modal for entering the entity's content.
  • Include custom entity definitions as part of the app's persisted state.

Screenshots

Checklist

• The PR follows the branch and commit styles outlined in the CONTRIBUTING doc
• (UI changes only) Linting and formatting checks pass locally with the new changes (npm run precommit from the ui directory)