fix(inputs.mysql): Avoid side-effects for TLS between plugin instances

[srebhan]

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

Currently, the plugin registers its TLS configuration as key "custom" to the MySQL driver. However, this registration is global to the driver package and therefor potential side-effects between multiple plugin instances arise if those instances register different TLS configurations.

This PR fixes the issue by replacing the "custom" key with a unique key while keeping the user-facing configuration the same. Furthermore, we move the initialization to the Init() function in order to save some cycles and avoid the necessity to frequently create random registrations.

[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
		static_linux_amd64.tar.gz

[powersj]

Thanks - while we don't have any integration tests that us TLS directly, they would cover the bulk of the path