[ENH] Make AutoETS Configurable

[alexbanwell1]

Reference Issues/PRs

Implements #3323

What does this implement/fix? Explain your changes.

Allows AutoETS to be configured to only select certain model configurations

Does your contribution introduce a new dependency? If yes, which one?

o

Any other comments?

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I have added the following labels to this PR based on the title: [ enhancement ].
I have added the following labels to this PR based on the changes made: [ forecasting ]. Feel free to change these if they do not properly represent the PR.

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[alexbanwell1]

Examples: "ANN", "AAN", "AAdN", "AAA", "AAdA", "MAdM"

Special codes:
- "ZZZ": Automatic selection using information criteria
- "ZAZ": Auto-select error and seasonal, additive trend only (default)

I have not included X, Y or C specified in the original proposal as I don't see how they are different from Ad, Md or Z respectively (or alternatively A, M or Z), but otherwise should be pretty much the substance of the proposal