Add file post-processing to Kotlin generators

[wing328]

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Description of the PR

• Add file post-processing to Kotlin generators (e.g. export KOTLIN_POST_PROCESS_FILE="/usr/local/bin/ktlint -F" with --enable-post-process-file using CLI)

3ca2aa8#diff-ed802784b178ca4a654ed7828c5eb0b8 shows the auto-format code done by ktlint

cc @jimschubert (2017/09) @dr4ke616 (2018/08)

[guenhter]

This is cool.
Should klint run then in the CI as well?

[wing328]

@guenhter very good question. At the moment, we don't do that because not all contributors (new or existing) have ktlint or other formatting tools (same version and setting) setup so we won't do it in our CIs at the moment.

[guenhter]

Ok, but this PR is then already great because we have the possibility to add klint.

[dr4ke616]

This is very useful indeed 👍

[wing328]

Thanks for reviewing. Merged it to master.

There's a minor issue with 2 files in the Kotlin cllient due to wildcard import: #1401

If anyone has time, please help fix it with a PR. Thanks.