wallet: make gap limit for change configurable

[Filiprogrammer]

Add the ability to configure the gap limit for change addresses while
retaining that value in the wallet database.

[SomberNight]

Hi. What is your use case for this?

note: related issue at #6440

[Filiprogrammer]

• Users may need to restore an existing wallet from a seed that includes transactions on change addresses beyond the current hardcoded gap limit. With the current setup, users have to manually change the gap limit for change addresses every time they open the wallet in Electrum. Introducing a configurable option that is saved in the wallet database alleviates this manual task, providing a more user-friendly experience and reducing the risk of oversight.
• Another point is consistency. Currently, receiving addresses allow for a configurable gap limit, while change addresses are limited by a hardcoded value.
• The ability to change the gap limit for change addresses via the CLI electrum changegaplimit <newlimit> --iknowwhatimdoing --change offers power users a convenient method for adjusting their wallet settings. And again, since it is already possible to change the gap limit for receiving addresses via the CLI, it makes sense to be consistent and have the same ability for change addresses.