Add public method validateBbanEntry to BbanStructure for BBAN entry validation by country

[PabloLec]

This PR addresses issue #155 and introduces a public API to validate specific parts of a BBAN based on the country. As described in the issue, a common use case might be validating an isolated bank code by checking its length and allowed characters, given the country.

I aimed to follow the project's coding style and implemented the public method and its logic in BbanStructure. While considering whether this should reside in IbanUtil, I concluded that a dedicated util class might not be necessary. BbanStructure already exposes public methods like forCountry and encapsulates the logic needed to validate entries for a specific country.

As part of this change, the validateBbanEntryCharacterType method was moved from IbanUtil to BbanStructure. IbanUtil now delegates BBAN entries validation to the newly created method in BbanStructure.

I kept exception handling consistent with the current codebase, continuing to use IbanFormatException as done in IbanUtil. However, there might be a discussion to have regarding whether it’s conceptually appropriate to throw an IbanFormatException when validating a single BBAN entry. I don’t have a strong opinion on this but wanted to point it out for consideration.