Change validate to instance method

[drnextgis]

Currently, Task.validate is marked as a class method, but there are scenarios where having access to the task instance within the validate method would be beneficial. For example, if a STAC item property name is passed as a task parameter and we need to validate that all input items possess that property, access to self.parameters would be useful. What do you think about changing validate to a regular instance method?

[ircwaves]

@drnextgis -- You're thinking along these lines #147 ? Seems reasonable to me, but is somewhat breaking-change, in that any subclasses out there which implement validate(cls, payload) will need to update to validate(self).

[drnextgis]

Yes, @ircwaves, this is exactly what I needed!