agent: Implement EAP identity and passphrase query

agent: Implement EAP identity and passphrase query

EAP credentials will be queried whenever connecting to an 802.1X network
unless EAP-TLS is used OR if service is provisioned via .config file.

Besides adding Identity field into the API, this patch also removes the
restriction to receive value for only one field per response. Now both
Identity and Passphrase can be received in one call.