Send agent version to Receivers

[otisg]

e.g. https://spm-receiver.sematext.com:443/spm-receiver/influx/v1/tags?v=3.0.0

[gr0]

In https://github.com/sematext/agent/pull/54 in addition to version (using v parameter) we've added the following headers:

• x-st-sta-version
• x-st-sta-builddate
• x-st-sta-buildversion

It is not needed right now, but we may use that in the future.

[megastef]

We use official influx client. We need to check if we can modify headers or URL. We could always add a tag :)