fix(env): apply convert_case to each nested key segment

[SAY-5]

When Environment::convert_case is combined with separator, the case conversion was applied to the whole dotted key at once, so convert_case quirks (e.g. UpperCamel not capitalizing words after a .) produced inconsistent segments like Otel.endpoint and broke overrides of file-sourced values. This converts each .-separated segment independently. Closes #693

[epage]

Interesting that convert_case considers . to be part of the word.

Note that we would prefer a fairly specific commit structure as noted in our contrib guide. See also https://epage.github.io/dev/pr-style/#c-test.

[SAY-5]

Split the change per the c-test guide: 06d760e is the test commit (should_panic = "missing configuration field" capturing today's behaviour) and c2920ac drops should_panic and adds the per-segment case conversion. Force-pushed.

[11alieberman]

@epage I just want to point out that this is the same exact change I made in the PR #746, which you closed immediately, so I'm not sure if you even looked at it.

You also mentioned in that comment that:

'The linked issue's status is S-triage. In general, we don't look at PRs and discourage the submission of them unless there is agreement on the problem and the proposed solution'

The issue's status was never changed, yet you looked at this PR and worked with the author to further it along instead of outright closing it.

I find that quite confusing and a bit disappointing from my side, to be honest.