[Feature] Support I/O schema field definition

[fmigneault]

PR opengeospatial/ogcapi-processes#172 introduces the requirements for schema provided in every input and output definition to represent the expected format of value when submitting inputs and retrieving output results.

This field can be easily added for basic types that are defined by Literal I/O and Complex I/O since the type of value is always known. The multiplicity via minOccurs and maxOccurs can also be used to formulate the corresponding JSON schema fields and whether the expected schema of value is a direct value or an array of them. The AllowedValues and SupportedValues definitions can be used to define the enum values of a string, if needed, similarly to how it is already resolved to form the CWL payload. Ranges, min/max int/float values require slightly more logic, but should also be solvable with corresponding WPS/CWL parameters.

When supporting this feature, the corresponding "Requirements Class" with "JSON Schema Level" should be added to the conformance list accordingly to how much we support (currently only Level 0).
https://github.com/pvretano/ogcapi-processes/blob/issue-170/core/clause_7b_jsonschema.adoc

update: Levels were removed, feature superseded by PR opengeospatial/ogcapi-processes#181

todo

• Update conformance list with JSON schema support
• Update I/O JSON generation in process description outputs to provide the computed schema definition
• For every Deploy I/O, the schema should be optional to remain backward compatible. If provided, it should be used as-is in the resulting process I/O (after validation that it is a proper OpenAPI schema definition).
• For every *Described process I/O, schema should be required (as per OGC-API). Missing (omitted) explicit specification from deployment, for builtin processes, or already existing processes, schema should be generated transparently from other I/O metadata to fulfill the requirement.
• Update FormatSchema to employ FileURL instead of ReferenceURL since other variants are not publicly accessible nor resolvable by schema validators ignored
  

  weaver/weaver/wps_restapi/swagger_definitions.py

  Lines 575 to 581 in 3ab577b

  	class FormatSchema(OneOfKeywordSchema):
  	_one_of = [
  	# pointer to a file or JSON schema relative item (as in OpenAPI definitions)
  	ReferenceURL(description="Reference where the schema definition can be retrieved to describe referenced data."),
  	# literal JSON schema, permissive since it can be anything
  	PermissiveMappingSchema(description="Explicit schema definition of the formatted reference data.")
  	]

• Remove schema = FormatSchema(missing=drop) definitions in all [Deploy]Format[MimeType|MediaType] definitions since the schema will be one level higher, in the I/O definition itself rather than under formats field.
  Left for backward compatibility if used by some input that provided more details about the expected schema for a given media-type input/output, but unused.
• Tests (items defined in opengeospatial/ogcapi-processes/core/abstract_tests/ogc-process-description)