Add FixedString Input Support to Base64 Coding Functions

Why it matters

This feature allows users to directly use FixedString type columns with base64 coding functions without needing to cast them to other types. It simplifies workflows and improves performance when working with fixed-length binary strings in base64 format.

How to use it

Use the existing base64 encoding and decoding functions (such as base64Encode and base64Decode) directly on columns or values of type FixedString. No additional casting is necessary.