MLECO-6540: Replaced local UTF-8 validator with simdutf is_utf8
- Removed custom static is_valid_utf8 implementation
- Added simdutf/is_utf8 as external dependency in config
- Replaced usage with new function which requires string length argument
Change-Id: Ia5ebe5e80b19fa1062695516a296f4369394eb3f Signed-off-by: Codrut Irimie Codrut.Irimie@arm.com