Transport configuration API

[offa]

An API to enable additional transport settings for HTTP.

Instead of using a configuration string it provides a programmatic way for configuration:

auto influxdb = influxdb::http("http://localhost:8086?db=test")
    .setProxy(…)
    .setTimeout(10_s)
    .setVerifyCert(false)
    .connect();

As an example this PR includes settings for timeout and certificate validation.

ToDo:

• Update docs
• Add missing API docs --> Review public API docs #187
• Deprecate Transports Proxy API ❓ --> Review Proxy API of Transport and TransportFactory #227
• Deprecate TransportFactorys Proxy API ❓ --> Review Proxy API of Transport and TransportFactory #227
• Update with master

Related:

• Transport options API #216
• Read timeout on InfluxDB-cxx #171
• Add option to allow Curl to not verify self signed certs #114

closes #225