Add system fields to input sources.

[gianm]

Main changes:

1. The SystemField enum defines system fields __file_uri, __file_path,
   and __file_bucket. They are associated with each input entity.

2. The SystemFieldInputSource interface can be added to any InputSource
   to make it system-field-capable. It sets up serialization of a list
   of configured systemFields in the JSON form of the input source, and
   provides a method getSystemFieldValue for computing the value of each
   system field. Cloud object, HDFS, HTTP, and Local now have this.

The SystemFieldInputSource isn't strictly necessary, since each input source could have implemented system fields internally in its own way. However, I think the interface is valuable because it helps ensure system fields are dealt with consistently, and because it provides a path to exposing system fields in SQL in a nice way. I think that ideally, they would be referenceable by name, but not participate in star expansion. AFAICT this would require a new Calcite feature. Relevant Calcite mailing list thread: https://lists.apache.org/thread/pnf3bx3jlrmv7q1q7jhwhsylrw4q5t20

Until then, system fields can be used in SQL without the planner's awareness: with EXTERN, add systemFields to the inputSource section, and add the system field names to the signature section.

[gianm]

The IT standard-its / integration-index-tests-middleManager (kafka-transactional-index) / kafka-transactional-index integration test (Compile=jdk8, Run=jdk8, Indexer=middleManager, Mysql=com.mysql.jdbc.Driver) is repeatedly failing when running ./it.sh ci due to this:

Error:  Failed to execute goal com.github.eirslett:frontend-maven-plugin:1.14.2:npm (npm-install) on project web-console: Failed to run task: 'npm ci' failed. org.apache.commons.exec.ExecuteException: Process exited with an error: 1 (Exit value: 1) -> [Help 1]
Error:  
Error:  To see the full stack trace of the errors, re-run Maven with the -e switch.
Error:  Re-run Maven using the -X switch to enable full debug logging.
Error:  
Error:  For more information about the errors and possible solutions, please read the following articles:
Error:  [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
Error:  
Error:  After correcting the problems, you can resume the build with the command
Error:    mvn <args> -rf :web-console
Error: Process completed with exit code 1.

I am not sure what's going on here but I don't think this is related to this PR. Should be safe to merge without this test passing.

[gianm]

I am not sure what's going on here but I don't think this is related to this PR. Should be safe to merge without this test passing.

I pushed a commit that simply merges master, which'll rerun all the tests. Hopefully that clears the problem.

[abhishekrb19]

LGTM!

[zachjsh]

🚀