Add openapi3.Schema.PropertyKeys

[diamondburned]

This commit adds the OrderedPropertyKeys method to the
openapi3.Schema:

OrderedPropertyKeys returns the keys of the properties in the order
they were defined. This is useful for generating code that needs to
iterate over the properties in a consistent order. If the keys could
not be extracted for some reason, then this method automatically
sorts the keys to be deterministic.

This is done via a temporary fork of the YAML-to-JSON transformation library.
It will not be ready until invopop/yaml#13 is merged.

[lzap]

I like this, the random order is driving me crazy.

No response so far in the yaml PR, I pinged them but this is already a second issue/pr without any response. It looks like to me that those folks were only interested in patching the CVE with no further development. Let's see if we get more information.