move validate from classmethod to instance method

[ircwaves]

Related Issue(s):

• Change validate to instance method #133

Proposed Changes:

1. push self._payload assignment up,
2. move self.validate(payload) -> self.validate(), with the latter inspecting the self._payload member.

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[ircwaves]

@philvarner and or @gadomski -- think this is warrants a ⚠️ Breaking Change in the log?

[pjhartzell]

@philvarner and or @gadomski -- think this is warrants a ⚠️ Breaking Change in the log?

I think so. It breaks existing packages that rely on stac-task.

[gadomski]

Yup, 👍🏼 to marking as breaking.

[gadomski]

LGTM, and I linked this to the referenced issue to close.

[pjhartzell]

Approved, subject to one fixup and comment.

Comment is that I have some uneasiness around why this was a classmethod to begin with. Are we missing an intended purpose? Perhaps a use more along the lines of a staticmethod where you could fling payloads at the class for validation? That said, I think the change is logical, and we can modify based on user feedback.