feat(secretstores): Add support for more plugins

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

This PR is based on #11232 and adds secret- and thus secret-store support for more plugins, namely:

• inputs.http_response for username and password
• inputs.kafka_consumer and outputs.kafka via common/kafka
• inputs.mqtt_consumer for username and password
• inputs.postgres for address
• inputs.postgres_extensible for address
• inputs.pgbouncer for address
• inputs.snmp_trap for sec_name, auth_password and priv_password
• inputs.sqlserver when used with auth_method = "connection_string" (default)
• inputs.vsphere for username and password as well as token authentication

[jdstrand]

I looked only at the changes on top of #11232 as they pertain to usage of the new secrets functionality within telegraf and it LGTM overall. Couple of nitpicks that you're free to ignore. Please think through the one question regarding Get() that is inline.

[telegraf-tiger]

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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
		static_linux_amd64.tar.gz