feat: port googleauth from multi_json to standard lib json

[quartzmo]

• googleauth is now guaranteed to use the standard library JSON engine
  natively, providing absolute consistency regardless of user configurations.
• Remove multi_json's possible dispatch overhead and lock/mutex contention.
• Slightly reduce overall memory footprint per request.
• Add pstore to gemspec for Ruby 4.0 compatibility.
• Tests under multi_json already enforced the use of the json gem.
• Wrap JSON::ParserError in Google::Auth::InitializationError at main entry points.

fixes: #572