Make the API configuration modular

[ansasaki]

Move the configuration of the API for each scope into the respective handler file.
Also, move the API configuration into a dedicated file, and make it modular.

This is a preparation to add support for multiple API versions.

[codecov]

Codecov Report

Attention: Patch coverage is 72.94118% with 69 lines in your changes missing coverage. Please review.

Project coverage is 60.01%. Comparing base (2f7b3ad) to head (9e528f1).
Report is 58 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-agent/src/api.rs	70.14%	20 Missing ⚠️
keylime-agent/src/keys_handler.rs	64.44%	16 Missing ⚠️
keylime-agent/src/agent_handler.rs	63.33%	11 Missing ⚠️
keylime-agent/src/notifications_handler.rs	67.64%	11 Missing ⚠️
keylime-agent/src/quotes_handler.rs	65.62%	11 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	60.01% <72.94%> (+2.42%)	⬆️
upstream-unit-tests	60.01% <72.94%> (+9.00%)	⬆️

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

Files with missing lines	Coverage Δ
keylime-agent/src/errors_handler.rs	55.81% <ø> (-0.35%)	⬇️
keylime-agent/src/main.rs	21.33% <100.00%> (-4.69%)	⬇️
keylime-agent/src/agent_handler.rs	59.57% <63.33%> (ø)
keylime-agent/src/notifications_handler.rs	66.66% <67.64%> (ø)
keylime-agent/src/quotes_handler.rs	52.29% <65.62%> (+1.42%)	⬆️
keylime-agent/src/keys_handler.rs	64.97% <64.44%> (-0.47%)	⬇️
keylime-agent/src/api.rs	70.14% <70.14%> (ø)

... and 8 files with indirect coverage changes

[THS-on]

Generally LGTM, just would like to have someone else that has more experince with actix to also take a look at it.

[ansasaki]

/packit retest-failed