Non-equidistant discrete dimensions for optimize_acqf_mixed_alternating (#2923)

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## Motivation

As dicussed in #2904, this is the first PR in planned series of PRs regarding adding additional functionality to `optimize_acqf_mixed_alternating`. In this one, non-equidistant discrete dimensions are introduced.

CC: TobyBoyne

### Have you read the [Contributing Guidelines on pull requests](https://github.com/pytorch/botorch/blob/main/CONTRIBUTING.md#pull-requests)?

Yes.

Pull Request resolved: #2923

Test Plan: Unit tests.

Reviewed By: Balandat

Differential Revision: D78901531

Pulled By: saitcakmak

fbshipit-source-id: c0b8b6953077465dc6cf669f2a2b776a98332d3f

Author: jduerholt