feat: add User-Agent header overwrite possibility

[yohan-couppey]

Proposed Changes

This is a proposal to add the possibility to overwrite the User-Agent field when we do requests.

Example of use :

c := client.NewClient( fmt.Sprintf("https://%s:%s", "influx_url", "influx_proxy_port"), "influx_token" )
// Ensures background processes finishes
defer c.Close()

c.HTTPService().SetUserAgent("custom_user_agent")

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate (doesn't seem appropriate)
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[vlastahajek]

Thanks for the PR.
However, there is no need to add overriding for any request header to client API.
There is general support for customizing HTTP requests via the Doer interface.

[yohan-couppey]

I agree that we can do it with the Doer interface, but it was more convenient as User-Agent is as common as the Authorization field and avoid a lot of code.

But I will continue to do it with the Doer interface then.

Thank you for your time, closing the PR.