fix(outputs.influxdb_v2): Allow overriding auth and agent headers

[DStrand1]

Summary

In #16147, the http header code for the http request was refactored, which led to a changing of the order of header application.

In v1.32.2:

1. "Hard-coded" headers (Authorization, User-Agent) were added to the headers
2. User-specified headers (config http_headers) were added, overwriting the "hard-coded" headers if manually specified

In v1.32.3+:

1. User-specified headers (config http_headers) were added to the headers
2. "Hard-coded" headers (Authorization, User-Agent) were added, overwriting the user-specified headers

In short, if an Authorization or User-Agent header was specified via the configuration http_headers in v1.32.3+, it would be overwritten by the default user agent and the secret store token. This PR reverts this change to be like v1.32.2 and older.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #16380

[telegraf-tiger]

Thanks so much for the pull request!
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[telegraf-tiger]

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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

[srebhan]

Thanks for the fix @DStrand1!