Add ETuple option to polynomial degrees

[Ap4sh]

This adds an as_ETuples keyword to degrees() for the two finite multivariate polynomial backends that currently expose different return types

The defaults are intentionally unchanged:

• MPolynomial_polydict.degrees() still returns an ETuple
• MPolynomial_libsingular.degrees() still returns a tuple

That keeps existing callers compatible while giving callers an explicit way to request either representation, similar to exponents(as_ETuples=...)

Part of #38632

Checks:

• ./sage -b
• ./sage -t --long src/sage/rings/polynomial/multi_polynomial_element.py src/sage/rings/polynomial/multi_polynomial_libsingular.pyx
• ./sage -tox -e relint -- src/sage/rings/polynomial/multi_polynomial_element.py src/sage/rings/polynomial/multi_polynomial_libsingular.pyx
• ./sage -tox -e rst -- src/sage/rings/polynomial/multi_polynomial_element.py src/sage/rings/polynomial/multi_polynomial_libsingular.pyx
• manual type checks for the defaults, as_ETuples=True, as_ETuples=False, and zero polynomials

📝 Checklist

• The title is concise and informative
• The description explains in detail what this PR is about
• I have linked a relevant issue or discussion
• I have created tests covering the changes
• I have updated the documentation and checked the documentation preview

⌛ Dependencies

None

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