Add an User-Agent header in the HTTPS request sent to the backend service

[CMCDragonkai]

As the diversification of SnapSearch client packages, the backend may
need this information to collect the usage statistics.

The recommend format of the HTTP User-Agent header are as follows,

a). User-Agent: snapsearch-client/<lang>
b). User-Agent: snapsearch-client-<lang>/<ver>
c). User-Agent: snapsearch-client (<lang>)
d). User-Agent: snapsearch-client (<lang>/<ver>)

[liuyu81]

I vote for "snapsearch-client (<language>/<version>)".