Add additional headers to auth request

[schmidtw]

This adds the set of headers from the convey header into the auth
request in a way that doesn't require a change to Themis.

[schmidtw]

I want to get some more thoughts from here: xmidt-org/xmidt#50 before we merge, but let me know what you think of the change @sadhyama .

[codecov]

Codecov Report

Merging #361 (441ecf7) into master (bdc2733) will decrease coverage by 0.06%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
- Coverage   93.88%   93.82%   -0.07%     
==========================================
  Files          53       53              
  Lines        9782     9783       +1     
==========================================
- Hits         9184     9179       -5     
- Misses        598      604       +6     

Flag	Coverage Δ
unittests	93.82% <92.85%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
tests/test_auth_token_more.c	62.50% <0.00%> (-4.17%)	⬇️
src/auth_token.c	99.22% <97.22%> (-0.03%)	⬇️
tests/test_auth_token.c	100.00% <100.00%> (ø)
tests/test_config.c	94.39% <100.00%> (+0.03%)	⬆️
tests/test_service_alive.c	84.61% <0.00%> (-1.93%)	⬇️
tests/test_client_list.c	95.77% <0.00%> (-1.41%)	⬇️

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[sadhyama]

I want to get some more thoughts from here: xmidt-org/xmidt#50 before we merge, but let me know what you think of the change @sadhyama .

PR changes looks fine @schmidtw . Thanks