gh-144001: Add ignorechars parameter to Base64 decoder

[moneebullah25]

Summary

• Add the ignorechars parameter to binascii.a2b_base64() and base64.b64decode()
• When provided, only characters in this set will be silently ignored during decoding
• Other non-base64 characters will cause a binascii.Error
• Similar to the existing ignorechars parameter in base64.a85decode()

This addresses the feature request in #144001 where the existing strict_mode/validate parameters are "all-or-nothing" - either all non-alphabet characters are rejected, or all are ignored. The new ignorechars parameter allows selective filtering (e.g., ignoring whitespace while rejecting other invalid characters).

API

# Ignore only whitespace, reject other invalid chars
base64.b64decode(b'YWJj\n', ignorechars=b'\n')  # OK: b'abc'
base64.b64decode(b'YWJj!', ignorechars=b'\n')   # Error

# Strict: reject all non-base64
base64.b64decode(b'YWJj\n', ignorechars=b'')    # Error

# Default behavior unchanged
base64.b64decode(b'YWJj\n')                      # OK: b'abc' (ignores all)

Closes #144001

• Issue: Add the ignorechars parameter in the Base64 decoder #144001

[picnixz]

Closing since we will likely prefer Serhiy's PR for that. Thanks for the suggestion.