Add CalculateBase64EscapeLen variations for the 3 base64 escaping methods (Base64Escape, WebSafeBase64Escape, WebSafeBase64EscapeWithPadding).

Also update CalculateBase64EscapedLen() documentation (it references outdated Base64Escape API), and reference it from WebSafeBase64Escape (to match Base64Escape).

PiperOrigin-RevId: 511647760
Change-Id: I7dee18645c2a779c0762bc71da75a4684ec2493f