fix(inputs.mysql): Handle custom TLS configs correctly

[srebhan]

Summary

It seems that the Mysql driver now more strictly checks TLS configuration names when parsing the DSN. Therefore, our current strategy of parsing the DSN and then conveniently replace the TLS configuration name in the struct fails as the custom alias is not a registered configuration.

This PR literally replaces the tls=custom part of the configuration with the actual name using a regular expression. This circumvents the parsing issue. The PR also adds unit-test to make sure the regexp is safe.

Checklist

• No AI generated code was used in this PR

Related issues

resolves #15490

[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