feat: add support for otel collector environment variables

[jharvey10]

PR Description

Adds support for converting environment variables in OpenTelemetry Collector config files to equivalent Alloy config syntax.

This is accomplished by:

• Adding a custom envprovider to override the default config parsing behavior of automatically replacing envvars with avlues
• Using a regular expression from here to search for environment variable references.
• Replacing environment variable references with sys.env("NAME_OF_ENV_VAR"), wrapped in a coalese to properly handle default values, if present in the otelcol config.

Which issue(s) this PR fixes

Closes #2947

Notes to the Reviewer

• Handling the envvars in this way means there's no way to distinguish between an original config of ${FOO} and $${FOO}. The logic will convert both of these to the sys.env format, even though this is not 100% congruent with how OpenTelemetry treats these values.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[CLAassistant]

All committers have signed the CLA.

[github-actions]

💻 Deploy preview deleted.

[clayton-cornell]

Docs are OK.

[wildum]

Nice work!

[jharvey10]

@mattdurham @wildum @dehaansa
If this looks good to all of you, would you mind giving me a merge? I would, but I believe I'm still in the probationary period.