Add codec-header config (closes #753)

[EddieWhi]

To set any header other than Authorization, the codec-header config parameter can be used.

What was changed

Added the code-header property using the same pattern as grpc-meta.
Grammatical change to description of grpc-meta (it's got punctuation when it gets formatted to a single line).
Refracted the conversion from config to stringMapHeadersProvider to a factory function. (Slight change to existing error message here).

Why?

I needed to set the cf-access-token header on codec server requests (see #753)

Checklist

1. Closes [Feature Request] Allow setting custom header on codec server requests #753

2. How was this tested:

• queried workflow history with codec server behind Cloudflare Access using the cf-access-token header.
• checked grpc-meta still works as before.
• checked validation of grpc-meta still works and codec-header validation works too.

Have NOT tested the codec-auth parameter, although it should still work (just because I haven't got a simple way to test and hope someone else does without me having to set something specific up).

3. Any docs updates needed?

Could update list of props in this section, although this section look out of date anyway (i.e. temporal env set -h does not return a full list of available properties).

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[cretz]

This looks great, let me get another team member approval and we'll merge (could be middle of next week due to holidays).