Add support for parameter Outputs of OGC API Processes

[francescoingv]

Overview

The parameter outputs is passed to the Manager, which in turn pass it to the Processor,
subject to the Processor being declared to support the parameter.

The conditional parameter passing to the Processor grant backward compatibility for already developed Processors.

Related Issue / discussion

Solve #1420,
Subset #1285

Additional information

This PR has a breaking signature in BaseManager.execute_process()
If needed, an additional PR could be made to have backward compatibility:
adding a supports_output property to the Manager and handling it as required (currently not in place as I think it may set a maintenance issue).

The backward compatibility of the Processor is given by a supports_outputs property of the BaseProcessor,
by default set to True.
To check the compatibility, the example processor HelloWorld has supports_outputs=True,
while EchoProcessor follow the old signature.

I think an error 501 should be raised on receiving the parameter outputs, and not having the Processor handling it.

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[francescoingv]

Sorry, I made some errors in the pull request process.
Will close and reopen a new one.