[FlightSQL] Inconsistent base64 encoding for arrow-go and arrow-rust

[niebayes]

For the handshake between a Flight SQL client and a Flight SQL server, the Arrow Rust implementation uses Base64 standard encoding with padding, while the Arrow Go implementation uses Base64 standard encoding without padding. Since these two are incompatible, our Flight SQL server cannot authenticate Go and Rust clients in a unified manner.

[niebayes]

Well, we have found a way for consistent decoding:

let my_base64 = GeneralPurpose::new(
            &alphabet::STANDARD,
            GeneralPurposeConfig::new().with_decode_padding_mode(DecodePaddingMode::Indifferent),
        );

We have to construct our own general-purpose decoder in the server side and set the decode padding mode to Indifferent so that the base64 decoder would allow the encoded string being padded or not padded.