Include common lambdas in generators

[MichalFoksa]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Various generator used same Mustache lambdas over and over. In this PR, DefaultCodegen registers set of following Mustache lambdas:

    .put("lowercase", new LowercaseLambda().generator(this))
    .put("uppercase", new UppercaseLambda())
    .put("titlecase", new TitlecaseLambda())
    .put("camelcase", new CamelCaseLambda().generator(this))
    .put("indented", new IndentedLambda())
    .put("indented_8", new IndentedLambda(8, " "))
    .put("indented_12", new IndentedLambda(12, " "))
    .put("indented_16", new IndentedLambda(16, " "));

Extension is done in such a way that each generator can:

• extend the list with additional lambdas

    @Override
    protected Builder<String, Lambda> addMustacheLambdas() {
        return super.addMustacheLambdas()
                .put("multiline_comment_4", new IndentedLambda(4, " ", "///"));
    }

• suppress all common lambdas by returning empty builder from addMustacheLambdas()

    @Override
    protected Builder<String, Lambda> addMustacheLambdas() {
        return new ImmutableMap.Builder<String, Mustache.Lambda>();
    }

NOTE: @jmini : I was not sure where to put new methods in DefaultCodegen. PLS give me an advised or feel free to update it directly.

Reviewers

• C++: @ravinikam , @stkrwork , @etherealjoy , @MartinDelille
• C#: @mandrean , @jimschubert
• F#: @nmfisher
• Kotlin: @jimschubert , @dr4ke616 , @karismann , @Zomzog
• Scala: @clasnake , @jimschubert , @shijinkui , @ramzimaalej

[jmini]

LGTM