Support for validating intent params

[danielwiehl]

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

Parameters that are passed along with intents can be marked as required, but no expected data type can be specified. Thus, on the provider side, it must always be checked whether the passed parameters satisfy the requirements.

Describe the solution you'd like

The platform should support the declaration of the expected data type for parameters. It should be possible to specify a JSON schema inline in the parameter definition or to reference an existing JSON schema.

I can imagine registering JSON schemas as capabilities that can be referenced in both parameter definitions and message payload validation. The platform should provide default JSON schema capabilities for primitive data types such as string, number, and boolean.

[danielwiehl]

See related issue #144.