Add support for kafka auth using SASL with PLAIN mechanism

[jaysonsantos]

Description

This adds variables for authentication and enabling simple ssl without specifying certificates and others

Motivation and Context

This should enable to use kafka with providers like confluent which uses sasl_plain over ssl.

How Has This Been Tested?

I've tested it against our cluster which requires authentication with and ssl

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation. (I am not sure here, does the docs only point to config/env.go?)
• I have updated the documentation accordingly. (Same as aboce)
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #412 into master will decrease coverage by 0.15%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
- Coverage   81.90%   81.75%   -0.16%     
==========================================
  Files          27       27              
  Lines        1603     1611       +8     
==========================================
+ Hits         1313     1317       +4     
- Misses        211      214       +3     
- Partials       79       80       +1     

Impacted Files	Coverage Δ
pkg/handler/data_recorder_kafka.go	73.49% <50.00%> (-2.51%)	⬇️
pkg/handler/export.go	69.81% <100.00%> (ø)

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[zhouzhuojie]

I think you can add a comment that FLAGR_RECORDER_KAFKA_USERNAME and FLAGR_RECORDER_KAFKA_PASSWORD will only work if FLAGR_RECORDER_KAFKA_SIMPLE_SSL=true

[marceloboeira]

Should we add something to the docs or the comment is enough?

[jaysonsantos]

Hey there, according to confluent docs you can also use SASL with PLAINTEXT which does not require SSL.

[zhouzhuojie]

Should we add something to the docs or the comment is enough?

The doc directly shows the content of this env.go file, so the comment here is good enough.

Also the naming of the env, what do you think of FLAGR_RECORDER_KAFKA_SASL_USERNAME and FLAGR_RECORDER_KAFKA_SASL_PASSWORD? Just to make it clear that it's using SASL

[jaysonsantos]

I've just rebased the code to force the tests to run as one of them got cancelled.

[zhouzhuojie]

Should we add something to the docs or the comment is enough?

The doc directly shows the content of this env.go file, so the comment here is good enough.

Also the naming of the env, what do you think of FLAGR_RECORDER_KAFKA_SASL_USERNAME and FLAGR_RECORDER_KAFKA_SASL_PASSWORD? Just to make it clear that it's using SASL

[jaysonsantos]

@zhouzhuojie ready for review again.