Avoid HTTP request body cloning.

[left-arm]

Consume body passed to post_message(body, url) and update callers.

Fold some long lines while I'm here to improve code readability.

[codecov-commenter]

Codecov Report

Merging #574 (f43bdd8) into main (0539551) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #574      +/-   ##
==========================================
- Coverage   61.82%   61.82%   -0.01%     
==========================================
  Files         174      174              
  Lines       18437    18443       +6     
  Branches     4014     4014              
==========================================
+ Hits        11399    11402       +3     
- Misses       2796     2802       +6     
+ Partials     4242     4239       -3     

Flag	Coverage Δ
integration-aries-vcx	50.66% <ø> (ø)
unittests-agency-client	25.85% <0.00%> (-0.15%)	⬇️
unittests-aries-vcx	55.61% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
agency_client/src/api/agent.rs	24.13% <0.00%> (ø)
agency_client/src/api/messaging.rs	0.00% <ø> (ø)
agency_client/src/httpclient.rs	7.01% <0.00%> (-0.83%)	⬇️
agency_client/src/internal/messaging.rs	30.20% <0.00%> (ø)
...vcx/src/protocols/issuance/issuer/state_machine.rs	71.41% <0.00%> (+0.12%)	⬆️
.../src/protocols/connection/inviter/state_machine.rs	70.68% <0.00%> (+0.34%)	⬆️

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