Allow encrypting and decrypting messages

package.json

snap.manifest.json

src/index.ts

src/rpc.ts

src/snap/decodeBuffer.ts

src/snap/dialogs/ConfirmBufferDecode.tsx

src/snap/dialogs/ConfirmBufferSign.tsx

src/snap/dialogs/ConfirmTransactionSign.tsx

src/snap/encodeBuffer.ts

src/snap/signTransaction.ts