feat: Add an option to specify headers

[kayoch1n]

This PR adds Header() option to the Client API, allowing users to associate key-value headers to tasks.

• Add Header(key, value string) Option function to client.go.
• Add parsing support in inspector.go.
• Add tests.

The underlying value of Header is [2]string. Inspector parses Header option by unmarshaling the JSON array argument back to [2]string

usage:

client.Enqueue(NewTask("send_email", data), Header("channel", "email"))

Follow-up to #1070 which added headers support to Task messages

[codecov]

Codecov Report

❌ Patch coverage is 62.96296% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.33%. Comparing base (4f00f52) to head (5216f1c).
⚠️ Report is 36 commits behind head on master.

Files with missing lines	Patch %	Lines
client.go	61.90%	8 Missing ⚠️
inspector.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
- Coverage   67.13%   63.33%   -3.81%     
==========================================
  Files          29       29              
  Lines        4300     5010     +710     
==========================================
+ Hits         2887     3173     +286     
- Misses       1135     1558     +423     
- Partials      278      279       +1     

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[kamikazechaser]

Could we rename this to something else like HeadersMap so that it is not confusing. #1070 also introduced a Headers method on Task.

[kamikazechaser]

Maybe another thing to document is when to use this vs what was introduced in #1070.