fix(config): Implement strict envvar handling to prevent insecure text replacement

[srebhan]

Summary

Implement a strict version of handling environment variables which uses the capabilities of TOML parsing instead of replacing the environment variables in the raw text. This comes at a cost of only being able to use environment variables
in string settings because the initial config file must be TOML conform.

Note

The strict handling requires to opt-in using the --strict-env-handling command-line option

Checklist

• No AI generated code was used in this PR
• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

[skartikey]

@srebhan Well designed and tested. Just one minor comment.

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.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_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

[daniel-falk]

Hi everyone! Is there any migration guide for this change? We are using environment variables both for numbers like metric_buffer_limit and for boolean flags, e.g. for the debug option in the agent configuration. What options are there to make this complaint without hard coding the option in the config file?