Pull out mixin for setting `X_pending` in acquisition functions

[esantorella]

Summary: Formerly, all subclasses of Acquisition had a method set_X_pending, even if they did not support pending points. This confused me while writing tests for optimizing acquisition functions.

• Pulled out a mixin XPendingMixin so that classes only have the method set_X_pending where it is functional and appropriate
• Removed "UnsupportedError" overrides from classes that no longer have a set_X_pending method
• Clarified which classes are supported in optimize_acqf and similar functions; added more precise type hints.

Differential Revision: D55843962

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D55843962

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D55843962