Agent class should be able to use keys added via `secretcli add`

**Option 1.** Use something similar to https://github.com/zealic/go2node to get private keys from the system keyring by the same means as `secretcli`.

* https://github.com/enigmampc/cosmos-sdk/blob/master/crypto/keyring/keyring.go
* https://github.com/enigmampc/cosmos-sdk/blob/master/client/keys/add.go
* https://github.com/enigmampc/SecretNetwork/blob/master/cmd/secretcli/main.go

This looks like it would necessitate a custom Go binary based on secretcli to be built for different platforms.

**Option 2.** The alternative would be to talk to the supported keyrings directly from JS:

```go
const (
BackendFile = "file"
BackendOS = "os"
BackendKWallet = "kwallet"
BackendPass = "pass"
BackendTest = "test"
BackendMemory = "memory"
)
```