chore: Fix linter findings for revive:exported in plugins/inputs/webhooks/*

[zak-pawel]

Summary

Address findings for revive:exported in plugins/inputs/webhooks.

As part of this effort for files from plugins/inputs/webhooks, the following actions were taken:

• Type names (const, var, struct, func, etc) were changed to unexported, wherever they didn't need to be exported.
• All remaining exported types were given comments in the appropriate form – this does not apply to exported methods that implement "known" plugin interfaces (Gather|Init|Start|Stop|SampleConfig|Parse|Add|Apply|Serialize|SerializeBatch|SetParser|SetParserFunc|GetState|SetState).
• The order of methods was organized (exported methods first, then unexported, with init at the very end).

It is only part of the bigger work (for issue: #15813).
After all findings of this type in whole project are handled, we can enable revive:exported rule in golangci-lint.

Checklist

• No AI generated code was used in this PR

[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 change the Telegraf binary size

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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

[srebhan]

Thanks @zak-pawel!