gh-90949: expose Expat API to tune exponential expansion protections (#139368)

gh-90949: expose Expat API to tune exponential expansion protections (#139368)

Expose the XML Expat 2.7.2 APIs to tune protections against
"billion laughs" [1] attacks.

The exposed APIs are available on Expat parsers, that is,
parsers created by `xml.parsers.expat.ParserCreate()`, as:

- `parser.SetBillionLaughsAttackProtectionActivationThreshold(threshold)`, and
- `parser.SetBillionLaughsAttackProtectionMaximumAmplification(max_factor)`.

This completes the work in f04bea44c37793561d753dd4ca6e7cd658137553,
and improves the existing related documentation.

[1]: https://en.wikipedia.org/wiki/Billion_laughs_attack