fix(agent): Add authorization and user-agent when watching remote configs

[srebhan]

Summary

When watching remote (http) configuration files we do issue a HEAD request to find out if the config changed. However, this request is currently missing the authentication header and therefore those requests fail if the endpoint requires authentication.
This PR adds the authentication header if a token is specified and also sends the Telegraf user agent in the request.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #16545

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

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Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz

[razvancicu]

Issue solved! 👍
Here are the logs from the HEAD requests, highlighting the Authorization header:

netutils  | 2025-02-24 16:09:49,936 - net_utils.telegraf_config.api.v1.snmp.ups - snmp_config():38: - INFO - Request headers: {'host': 'netutils:8099', 'user-agent': 'Telegraf/1.34.0-2e157ad0 Go/1.23.5', 'authorization': '****'}