Change customTemplates Map key to be type String

[velo]

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Description

Related to #1048

Only realized I was testing the wrong SNAPSHOT when I change the version from 5.6.1-SNAPSHOT to 5.7.0-SNAPSHOT.

Can't use the enum as key on the map, as build tools will inject strings.

Also did a minor improvement that allow to setup maven like this:

<configuration>
  <customTemplates>
    <TYPE>src/main/resource/type.ftl</TYPE>
  </customTemplates>
<configuration>

Before template need to be on classpath and was whole lot more complicated

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Changes were made to:

• Codegen library - Java
• Codegen library - Kotlin
• Codegen library - Scala
• Maven plugin
• Gradle plugin
• SBT plugin

[kobylynskyi]

@velo changes look good. Can you please fix the build? (checkstyle issues)

[velo]

@velo changes look good. Can you please fix the build? (checkstyle issues)

BTW, can I run checkstyle locally?

I enabled the plugin, but seems I don't get the same warnings...

[kobylynskyi]

@velo yes, you can. There are plugins for different IDEs and you can import the rules from the following folder: config/check-style