Introduce checkstyle to enforce conventions

[masayag]

What this PR does / why we need it:
We'd like to make sure our code base is aligned with its conventions.

Which issue(s) this PR fixes :
Fixes #FLPATH-347

Change type

• New feature
• Bug fix
• Unit tests
• Integration tests
• CI
• Documentation
• Auto generated SDK code

Impacted services

• Workflow Service
• Notification Service

Checklist

• Subject and description added to both, commit and PR.
• Relevant issues have been referenced.

[masayag]

This PR is going to heavily fail since our code base doesn't entirely match the convention of spring.
For that, we'll have to disable the rules and gradually add them when fixed.
I suggest enabling the checkstyle plugin per module to make the work sustainable and not too overwhelming.
I'll introduce a set of rules to be excluded in next commits of this PR.

[pkliczewski]

/lgtm

[nirarg]

/approve

[openshift-ci]

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[gciavarrini]

@masayag may I suggest updating the Makefile target format-files?

[masayag]

@masayag may I suggest updating the Makefile target format-files?

@gciavarrini as the name and the action suggests - format-files changes the files.
with checkstyle, all can be done is check action, not modify the files. Hence it doesn't fit.