nDPI feat: replace old readable string extraction with new filtered multi-string support

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[ X ] I have signed the ntop Contributor License Agreement at https://github.com/ntop/legal/blob/main/individual-contributor-licence-agreement.md [ X ] I have read the contributing guidelines at https://github.com/ntop/nDPI/blob/dev/CONTRIBUTING.md [ ] I have updated the documentation (in doc/) to reflect the changes made (if applicable)

Link to the related issue: References https://github.com/ntop/nDPI/issues/2708

Describe changes:

Feat: add extract_readable_strings with advanced filtering; refactor string handling

Introduces a new function to retrieve multiple human-readable substrings from a payload, with optional filter callback support (e.g., for FQDN detection).
Replaces the old function with the new approach.
Increases size from 32 to 128 bytes to support longer string concatenations.
Renames all occurrences of to the correct spelling .
Replaces all calls to with , reflecting the new logic.