adding convert_cell to read_excel
- [x ] closes #49872
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

The converters do the convertion after the reading (i'm not sure of that) and doesn't have access to the cell format. 
In my case i have a  convert a cell with the number 2345, with numeric_format 00000 to the string '02345'. In Excel you see this as a number with leading zeros, wich isn't valid python. If a use a converter i will convert the number 2345 to the string '2345'.
I'm working normalizing financial documents, and in a lot of cases they use format as a data, for example another color for negative numbers(not as condicional, as simple formatted), and things like that. I don't know if this is useful for a lot of people. Another option, less agressive is just add in the read_excel documentation an explanation that, in the case you want to access cell format you have to override _convert_cell(cell). I'm new in the community, is better have the discussion of the pertinence of the request in the pull request, the issue or the slack? Discussion should be on the issue. Please wait with an implementation until there is agreement Thanks for the pull request, but agreed discussion is needed in the original issue first before moving forward with a PR, so closing until a decision has been made there