feat: Allow setting of application name to the User-Agent header value

[vlastahajek]

Closes #356

Proposed Changes

Added new option for setting the application name. Application name is then added to the end of Go client version string: E.g. Influxdb-client-go/2.10.0 (linux; amd64) IoTHub/1.10

• Added also example how to completely overwrite User-Agent header value using the Doer interface

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• Tests pass
• Commit messages are in semantic format

[vlastahajek]

@lesam, As you noted in issue #356, there should be an annoying log message, if the application name is unset.
The current state of the logging mechanism is that only errors are shown by default. Thus writing such a message which would be always visible in the log requires writing it as an error.
But, it will introduce this annoying message to all users. This doesn't look like friendly instruction for a new feature. Writing it as a warning seems to be better.
WDYT?

[lesam]

It seems reasonable to start with a warning, put it in the release notes/docs, and then maybe upgrade it to an error message in the next version?

@dgnorton for thoughts as well.

[lesam]

Just leaving it as a warning forever is not sufficient, since it will never be noticed.

[vlastahajek]

@lesam, @dgnorton, @powersj, the release is just outside the door, we have to decide if to stay with the error level or change the message to warning.

[vlastahajek]

We had an internal discussion in Bonitoo and we are in favor of a warning because such a small feature should not bug users.

[bednar]

I prefer to use warning and add a warning to CHANGELOG.md that it will change to error in a future version of the client.

[bednar]

LGTM 🚀