Add PHP 7.4 runtime

[akrabat]

Description

Add PHP 7.4 runtime:

• Update list of runtimes.
• Update Swagger description
• Update docs
• Add unicode.tests file

Note that this requires apache/openwhisk-runtime-php#79 to be merged first.

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
• Invoker
• Intrinsic actions (e.g., sequences, conductors)
• Data stores (e.g., CouchDB)
• Tests
• Deployment
• CLI
• General tooling
• Documentation

(Note: I'm not sure which section updating a runtime falls into!)

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.

[akrabat]

Note: I'm unsure how the text files in test/dat/actions/unicode.test/ are used, but added a PHP 7.4 one anyway.

[akrabat]

The tests have failed on "Kafka connector should send and receive a kafka message even after shutdown one of instances". I assume this is because the PHP 7.4. runtime isn't published yet?

[codecov-io]

Codecov Report

Merging #4767 into master will decrease coverage by 6.49%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #4767     +/-   ##
=========================================
- Coverage   84.85%   78.36%   -6.5%     
=========================================
  Files         198      198             
  Lines        8836     8843      +7     
  Branches      602      602             
=========================================
- Hits         7498     6930    -568     
- Misses       1338     1913    +575

Impacted Files	Coverage Δ
...core/database/cosmosdb/RxObservableImplicits.scala	0% <0%> (-100%)	⬇️
...enwhisk/connector/kafka/KamonMetricsReporter.scala	0% <0%> (-100%)	⬇️
...e/database/cosmosdb/cache/ChangeFeedConsumer.scala	0% <0%> (-100%)	⬇️
...ore/database/cosmosdb/cache/CacheInvalidator.scala	0% <0%> (-100%)	⬇️
...core/database/cosmosdb/CosmosDBArtifactStore.scala	0% <0%> (-96.61%)	⬇️
...sk/core/database/cosmosdb/CosmosDBViewMapper.scala	0% <0%> (-93.85%)	⬇️
...tabase/cosmosdb/cache/CacheInvalidatorConfig.scala	0% <0%> (-92.31%)	⬇️
...e/database/cosmosdb/cache/KafkaEventProducer.scala	0% <0%> (-77.78%)	⬇️
...whisk/core/database/cosmosdb/CosmosDBSupport.scala	0% <0%> (-74.08%)	⬇️
...abase/cosmosdb/CosmosDBArtifactStoreProvider.scala	7.69% <0%> (-50%)	⬇️
... and 11 more

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