BUG: #29049 make holiday support offsets of offsets
- [x] closes #29049 (Replace xxxx with the GitHub issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature.

The summing of offsets is generally not possible, so the bug is fixed by simply flattening the asymmetric list of lists of offsets that one might easily end up with when defining holidays in relation to each other.

In theory it is possible to sum consecutive `DataOffset(n, weekday=None)` in the offset list, but I did not want to introduce even more complexity for such little gain.


Allowing asymmetrical lists of lists and their element dtypes as parameters is not very pretty, but the only solution to this problem that does not involve a major refactoring of the offset classes to allow representation of lists of offsets in a single composite offset e.g. ["TUE(1)", 3]. Summing of composite offsets is not possible because additions between weekday and regular day shift offsets are not associative, not distributive, and not commutative.

Another option would be to just enhance the error message given to the end user when trying to chain offsets in the manner shown in the issue and let them do the aggregation to a simple list of offsets themselves.
Thanks @dontgoto  Thank you for the input @mroeschke 