Add case-insensitive fallback for response header logging

[ozangoktan]

Adds a fallback case-insensitive mode of response header logging. It is slower than matching by exact string, as we have to process response headers, but we only do that if the exact string match fails.

Enforces response header logging keys to be case-insensitive as per the standard.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.54%. Comparing base (44def6b) to head (aff04d8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #150 +/- ## ========================================== + Coverage 77.29% 77.54% +0.24% ========================================== Files 9 9 Lines 674 677 +3 Branches 40 38 -2 ========================================== + Hits 521 525 +4 Misses 126 126 + Partials 27 26 -1 ``` | [Files](https://app.codecov.io/gh/cognitedata/oryx/pull/150?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cognitedata) | Coverage Δ | | |---|---|---| | [src/Logging.fs](https://app.codecov.io/gh/cognitedata/oryx/pull/150?src=pr&el=tree&filepath=src%2FLogging.fs&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cognitedata#diff-c3JjL0xvZ2dpbmcuZnM=) | `96.47% <100.00%> (+1.34%)` | :arrow_up: |