Add support for custom headers with dash in their names

[sjoubert]

• Request both the class name and header name in the macro as lots of headers have a '-' but their name can't also be used for the class
• Prefix the new macro with PISTACHE_ to avoid potential name conflicts

[sjoubert]

I'm not sure what is your backward compatibility policy. In doubt I've kept the CUSTOM_HEADER macro, but it should probably be deprecated in favor of the new one. I don't really know how to mark a macro as deprecated though.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (e884d6d) 78.60% compared to head (74f5344) 78.60%.

:exclamation: Current head 74f5344 differs from pull request most recent head 41676ee. Consider uploading reports for the commit 41676ee to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1136 +/- ## ======================================= Coverage 78.60% 78.60% ======================================= Files 53 53 Lines 6884 6884 ======================================= Hits 5411 5411 Misses 1473 1473 ``` | [Impacted Files](https://app.codecov.io/gh/pistacheio/pistache/pull/1136?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [include/pistache/http\_header.h](https://app.codecov.io/gh/pistacheio/pistache/pull/1136?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-aW5jbHVkZS9waXN0YWNoZS9odHRwX2hlYWRlci5o) | `90.62% <ø> (ø)` | |

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[Tachi107]

Thanks for the patch, @sjoubert!