Forward header from a trigger to actions.

[style95]

Description

This is to forward headers delivered to triggers to actions.
This is useful when an event source sends some information via headers.
A good example could be the X-Github-Event header in Github APIs.

Currently, this kind of information is dropped.

Related issue and scope

• I opened an issue to propose and discuss this change (#????)

My changes affect the following components

• API
• Controller
• Message Bus (e.g., Kafka)
• Loadbalancer
• Scheduler
• Invoker
• Intrinsic actions (e.g., sequences, conductors)
• Data stores (e.g., CouchDB)
• Tests
• Deployment
• CLI
• General tooling
• Documentation

Types of changes

• Bug fix (generally a non-breaking change which closes an issue).
• Enhancement or new feature (adds new functionality).
• Breaking change (a bug fix or enhancement which changes existing behavior).

Checklist:

• I signed an Apache CLA.
• I reviewed the style guides and followed the recommendations (Travis CI will check :).
• I added tests to cover my changes.
• My changes require further changes to the documentation.
• I updated the documentation where necessary.

[codecov-commenter]

Codecov Report

Merging #5273 (9afa2a4) into master (639c4a9) will decrease coverage by 14.62%.
The diff coverage is 88.88%.

@@             Coverage Diff             @@
##           master    #5273       +/-   ##
===========================================
- Coverage   80.07%   65.45%   -14.63%     
===========================================
  Files         238      238               
  Lines       14084    14130       +46     
  Branches      588      582        -6     
===========================================
- Hits        11278     9249     -2029     
- Misses       2806     4881     +2075     

Impacted Files	Coverage Δ
...rg/apache/openwhisk/core/controller/Triggers.scala	81.37% <88.88%> (+0.35%)	⬆️
...nwhisk/core/scheduler/queue/ContainerCounter.scala	0.00% <0.00%> (-100.00%)	⬇️
...hisk/core/scheduler/message/ContainerMessage.scala	0.00% <0.00%> (-100.00%)	⬇️
...core/database/cosmosdb/RxObservableImplicits.scala	0.00% <0.00%> (-100.00%)	⬇️
...ore/database/cosmosdb/cache/CacheInvalidator.scala	0.00% <0.00%> (-100.00%)	⬇️
...e/database/cosmosdb/cache/ChangeFeedConsumer.scala	0.00% <0.00%> (-100.00%)	⬇️
...core/scheduler/queue/SchedulingDecisionMaker.scala	0.00% <0.00%> (-98.80%)	⬇️
...sk/core/scheduler/container/ContainerManager.scala	0.00% <0.00%> (-96.90%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0.00% <0.00%> (-95.85%)	⬇️
.../openwhisk/core/scheduler/FPCSchedulerServer.scala	0.00% <0.00%> (-95.84%)	⬇️
... and 39 more

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[upgle]

Other similar test codes seem to contain apache header.

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.