Support base64 encoding for encrypt/decrypt API

[kthomas]

Currently, the encrypt/decrypt endpoints only support hex encoding.

We should consider adding an encoding parameter to the EncryptDecryptRequestResponse in provide-go.

// EncryptDecryptRequestResponse contains the data (i.e., encrypted or decrypted) and an optional nonce
type EncryptDecryptRequestResponse struct {
	Data  string  `json:"data"`
	Encoding  string  `json:"encoding"` // we should add support for this...
	Nonce *string `json:"nonce,omitempty"`
}