fix(agent): Handle float time with fractions of seconds correctly

[srebhan]

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

In the course of #12490 I found an issue when specifying a duration with fractions of seconds as a plain floating point value e.g. timeout = 1.5, where you would expect the timeout to be 1500 milliseconds. However the current code will just floor that value to 1 second.

This PR fixes the handling of plain floating point values that contain fractions of seconds and also gets rid of the necessity for TOML quotation handling by using the UnmarshalText interface.

[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.

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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

[powersj]

Making changes to this area of code concerns me, but the tests and additional tests (thanks) do look correct I believe, so thanks for this!