Fix dummy values in gRPC message sent to RC xApp

TS xApp did not populate ran_name, plmn_id, and e2_node_id
in gRPC messages sent to RC xApp. These fields were always
set up with dummy values.

This change maps the nodeb's information from the handoff target cell,
and populates the required information in corresponding fields.
This change also adds a generic REST client API to send HTTP requests,
and allows the E2 Manager endpoint used by TS to changed using the
environment variable SERVICE_E2MGR_HTTP_BASE_URL.

Issue-ID: RICAPP-187

Signed-off-by: Alexandre Huff <alexandrehuff@utfpr.edu.br>
Change-Id: I45acb87ad2391199658d41457919279e46c97fdf