Support cert/private keys as string in connection profile

The connection profile (`network.json`) supports specifying certs and private keys as a path.

Similarly to what the NodeJS sdk client provides (https://github.com/hyperledger/fabric-sdk-node/blob/master/test/fixtures/profiles/network.json#L49) it would be useful to allow specifying the certificates contents directly as strings, e.g.

```
"users": {
"Admin": {
"cert_pem": "-----BEGIN CERTIFICATE-----\nMIIB+zCCAaGgAwIBAgIUSFaxTErz/QK2xfiFuHR3km8cNFowCgYIKoZIzj0EAwIw\nWjELMAkGA1UEBhMCVVMxFzAVBgNVBAgTDk5vcn ... n-----END CERTIFICATE-----"
"private_key_pem": "-----BEGIN PRIVATE KEY-----\nMIGHAgEAM ... 5zIHXyi2yB+hWxJ\n-----END PRIVATE KEY-----"
}
}

[...]

"tlsCACerts": {
"pem": "-----BEGIN CERTIFICATE-----\nMIIB+zCCAaGgAwIBAgIUSFaxTErz/QK2xfiFuHR3km8cNFowCgYIKoZIzj0EAwIw\nWjELMAkGA1UEBhMCVVMxFzAVBgNVBAgTDk5vcn ... n-----END CERTIFICATE-----"
},

```