GetTiDBConnection gets the TiDB connection stored in the gin context by `MWConnectTiDB` middleware.

MWConnectTiDB creates a middleware that attaches TiDB connection to the context, according to the identity information attached in the context.

MWHandleErrors creates a middleware that turns (last) error in the context into an APIError json response.

NewJWTString create a JWT string by given data.

ParseJWTString parse the JWT string and return the raw data.

TakeTiDBConnection takes out the TiDB connection stored in the gin context by `MWConnectTiDB` middleware.

The key that attached the SessionUser in the gin Context.

Claims is a struct that will be encoded to a JWT.