Add Passing Client Headers section to rest-client.adoc

[pehala]

This section can be useful for passing Authorization headers in rest-client requests. The text is based on javadoc here https://github.com/eclipse/microprofile-rest-client/blob/master/api/src/main/java/org/eclipse/microprofile/rest/client/ext/DefaultClientHeadersFactoryImpl.java.

[michalszynkiewicz]

@pehala thank you for your PR :)
I'm wondering what's the best way of structuring this info. The guide already has a section that talks a bit about headers: https://quarkus.io/guides/rest-client#custom-headers-support

It mentions the @RegisterClientHeaders annotation. I think the info from this PR and what's in this section should be merged. It wouldn't hurt to add some info about @ClientHeaderParam btw...

[pehala]

@michalszynkiewicz The main reason why I even have this MR was that I needed to pass Authorization and it was really hard to find some info about how to do it. The thing that was missing the most was the property value which is needed if you don't specify the factory, e.g. you use the default one, which in my opinion will suffice for many cases.

If that is ok with you, I can ditch this new section and incorporate only that thing into the existing one.

[michalszynkiewicz]

👍🏻

[gsmet]

Could we finalize this PR? Thanks!

[gastaldi]

+1 to move that section near (or inside) https://quarkus.io/guides/rest-client#custom-headers-support, for consistency

[pehala]

Sorry for the delay, I moved the section inside https://quarkus.io/guides/rest-client#custom-headers-support.

Is it now ok?

[gastaldi]

Could you squash the commits in this PR to a single commit too?

[pehala]

Squashed and rebased

[gastaldi]

LGTM