refactor: common task properties:

[jstirnaman]

• add TaskProperties.yml that defines schemas for properties.
• in _schemas.yml, define NamedSchemas for the properties so they can be referenced elsewhere.
• replace duplicated properties with to the schemas in TaskProperties.yml ~~- - remove cloud TaskCreateRequest, use the common schema change cloud POST /tasks to use oneOf TaskCreateRequest or TaskWithScriptCreateRequest to define the difference in the schema, not just in the description.~~
• cleanup descriptions.

~~Docs example for `oneOf' ~~

In request examples, we can do this:

[jstirnaman]

@glinton Please review and let me know if splitting the cloud schema like this will work OK. Also, if the properties schema works as a pattern. I don't like having to define each property as a schema individually in _schemas.yml, but that's the only way I could make it work.