Predictable and customizable GUI generation in `cuiman`

[forman]

Is your feature request related to a problem? Please describe.

Currently, the GUI generation in cuiman is quite simple and mainly relies on standard properties defined in OGC API Processes descriptions and the inputs'/outputs' JSON Schemas, if any.

• It lacks a defined way to extract extra metadata for a predictable and customizable presentation of inputs (and outputs) in the generated GUIs.
• It does currently not handle schema type array (Add GUI for process inputs of type "array" #37) and object (Add GUI for process inputs of type "object" #38)
• It does currently not handle important schema elements such as oneOf, anyOf, allOf, discriminator, $ref`

Describe the solution you'd like

A clean OGC API Processes UI client pipeline would look like this

    Process description
           ↓
    Parameter model
           ↓
    JSON Schema merge
           ↓
    UI metadata merge
           ↓
    Field model
           ↓
    Panels

which cleanly keeps architectural layers separated:

 - Semantics, Validation  <-->   JSON Schema
 - Metadata  <-->   OGC input/output descriptions
 - Presentation  <-->  Extensions fields `"x-ui"` / `"ui"` in the above

Checklist:

• The implementation of the CUI-generation core should follow the new GUI-generation reference guide, which details the pipeline above.
• The hack ClientConfig.is_advanced_input() should then be replaced by the new solution.
• It should be clearly documented what kind of GUI customisations are recognized and supported by cuiman.

Related

• PRs: Support tooltips #56, Various Cuiman GUI enhancements #43, Add custom model fields / filter inputs in UI  #30
• Issues: Add GUI for process inputs of type "array" #37, Add GUI for process inputs of type "object" #38