feat(parsers.xpath): Add support for returning underlying data-types

[srebhan]

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

With this PR it is now possible to retrieve the native data-types for fields without explicit conversion for data-formats that inherently know the type. These formats are JSON (xpath_json), MessagePack (xpath_msgpack) and Protocol-buffers (xpath_protobuf). If you enable native types by setting xpath_native_types = true and query a float field (e.g. value = "//myvalue) you will now get a float64 value for that field without explicit conversion required before (value = "number(//myvalue)). This can be helpful when dealing with multiple message types, sharing the same field name, but do have different types...

[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_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