feat(router): make header matching rules case insensitive

[df-wg]

Motivation and Context

Many users have found themselves in confusing places when they've tried to apply our header propagation rules, because they neglected to use the case insensitive prefix to the regex.

HTTP Header names are by RFC case insensitive, so there should be no need to explicitly declare them as case insensitive.

Any case where we were using the <header>.Get(HEADER) was by its nature case insensitive, but this specifically needed to fix the header matching rules

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-7b532fc7d8a5729cb8de377bd15385787e4b94c9

[alepane21]

I was initially concerned that prepending the prefix to the regexp might break some existing patterns. However, I couldn’t identify any issues during testing. In the worst case, a previously non-functional rule might start working unexpectedly, but this risk seems minimal and non-blocking.