Capitalize all variant names when configuration specifies so in Go

[hculea]

Before this MR, when generating Go types with algebraic enums and specifying uppercase acronyms, this would happen:

func (i ItemFieldDetails) OTP() *OtpFieldDetails {
	res, _ := i.content.(*OtpFieldDetails)
	return res
}

// Additional attributes for OTP fields.
type OTPFieldDetails struct {
	// The OTP code, if successfully computed
	Code *string `json:"code,omitempty"`
	// The error message, if the OTP code could not be computed
	ErrorMessage *string `json:"error_message,omitempty"`
}

The uppercasing was not enforced consistently, causing the Go types to error because of their different capitalization: OTPFieldDetails vs OtpFieldDetails.

This MR fixes this.