feat(inputs.x509_cert): add smtp protocol

[skillor]

Required for all PRs:

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

resolves #7679 x509_cert: add support for SMTP certificates over STARTTLS

I picked up the changes from #7680.

Not too sure about the tests, because there is no easy possibility to test with a smtp server.

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.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_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
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

[srebhan]

Looks good to me. Thanks for the contribution @skillor!

[chrism417]

I ran this in our kubernetes cluster, worked great!

From prometheus:

x509_cert_verification_code{common_name="ISRG Root X1", container="telegraf", country="US", endpoint="http", environment="dev", issuer_common_name="DST Root CA X3", job="telegraf", namespace="monitoring", organization="Internet Security Research Group", public_key_algorithm="RSA", serial_number="4001772137d4e942b8ee76aa3c640ab7", service="telegraf", signature_algorithm="SHA256-RSA", source="smtp://redacted:25", telegraf="true", verification="invalid", verification_error="x509: certificate has expired or is not yet valid: current time 2022-06-15T17:33:31Z is after 2021-09-30T14:01:15Z"}