Allow user provided metadata to override default metadata

[dbhobbs]

This PR introduces code that will join duplicate headers into a comma-separated list, if allowed. Otherwise, it will prefer the user supplied value in the case that a default value was provided. It also moves required pseudo-headers into a separate macro which can't be overridden.

Section 4.2 of RFC2616 lays out some rules around multiple message headers.

Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.

Allowing duplicate headers like Content-Type would change the semantic meaning of the request so, in this PR, only the first header processed is kept. This allows user supplied metadata/headers to override the defaults, but prevents duplicates from being sent.

Addresses issue #60.

I've tested this code in my company's backend system and can successfully publish messages to Google Cloud PubSub.

[codecov]

Codecov Report

Merging #62 (6496cda) into main (0166760) will decrease coverage by 0.21%.
The diff coverage is 27.58%.

❗ Current head 6496cda differs from pull request most recent head 3c2ec9e. Consider uploading reports for the commit 3c2ec9e to get more accurate results

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   38.99%   38.78%   -0.22%     
==========================================
  Files          28       28              
  Lines        2090     2117      +27     
==========================================
+ Hits          815      821       +6     
- Misses       1275     1296      +21     

Impacted Files	Coverage Δ
src/grpcbox_client_stream.erl	56.86% <27.58%> (-12.48%)	⬇️

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[tsloughter]

Damn, its been years. I want to merge this functionality but the merge is now hairy. I may have to tweak it to get it working with the latest main and open a new PR.