Update Apigee Fault Codes for ARC

[kansalarchit]

https://github.com/apigee/apigee-remote-service-envoy/issues/371

Adding x-apigee-fault-code values for key management and JWT based authn and authz as well as other internal errors for ARC.

Notes:

• We cannot get more fine-grained fault-codes for key management yet because of how MP communicates the errors. MP only says invalid apikey. We need to either expand the existing handler or wait for the ACMS apis for getting more details, both of which are beyond the scope of this PR.
• Adding a default case to the Key management error switch-case as per discussion with @emschwar to ensure new errors in future get handled more gracefully.
• I am not adding / updating the unit tests because looks like we only validate whether the error was raised and not the exact details such as rpc code or fault string for the errors. Let me know if that should change.

[codecov[bot]]

Codecov Report

Merging #372 (66b1d56) into master (6ae5256) will increase coverage by 1%. The diff coverage is 93%.

@@          Coverage Diff          @@
##           master   #372   +/-   ##
=====================================
+ Coverage      91%    91%   +1%     
=====================================
  Files          15     15           
  Lines        1928   1960   +32     
=====================================
+ Hits         1746   1776   +30     
- Misses        102    104    +2     
  Partials       80     80           

Impacted Files	Coverage Δ
server/authorization.go	93% <80%> (-<1%)	:arrow_down:
config/env_spec_request.go	93% <98%> (+1%)	:arrow_up:
fault/fault.go	100% <100%> (ø)

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