Fix schema validation #16, #17

[m-mohr]

• Fixes JSON Schema to check providers properly Processing properties are not required on providers #8
• Fixes JSON Schema to check field requirements properly and everywhere (except summaries, as mentioned in the docs) Bug: Invalid collection properties are treated as valid if there are some valid values #16
• Clarifies that Items are required to contain one processing field.

I made quite a number of tests to check that the JSON Schema works in different scenarios and couldn't find any issues.

May supersede a couple of existing PRs (#6, #9) as I think the JSON Schemas in those PRs don't fully capture all issues and it could avoid the unit tests, it seems better to have tests in JSON Schemas so that other tooling (PySTAC, validators) can make use of it. @l0b0

I hope that we can release a 1.1.0 of the extension after this has been merged.

[emmanuelmathot]

LGTM as long as it validates the examples. To be honest, I tried to review the new schemas but I must admit it goes beyond my expertise.