[Base64Utils] Removed regex to validate base64

[CastagnaIT]

Description

with very large strings regex can cause this error:

regex_error(error_stack): There was insufficient memory to determine whether the regular expression could match the specified character sequence

seem to be a common c++ regex library problem that happen when a pattern is complex, so has been replaced with similar code implementation

Motivation and context

fix #1681

How has this been tested?

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly