Fix AbstractJavaCodegen ability to customize container types

[dmivankov]

Fix for #13549

After #5466 java codegen emits Set for array with uniqueItems=true which has its downsides

• easy to lose track of element order (it can be important)
• easy to lose ordering by switching between java Set flavors (HashSet vs LinkedHashSet) somewhere down the line
• bigger divergence from original json types (either array or object)

Luckily types can be customized/overriden by extending the generator:

• extend JavaClientCodegen for example
• in constructor do

    super();
    // import/interface type
    typeMapping().put("set", typeMapping().get("array"));
    // concrete type
    instantiationTypes().put("set", instantiationTypes().get("array"));

But sometimes it won't work because of missing ArrayList import, can be fixed by also doing

    // workaround for https://github.com/OpenAPITools/openapi-generator/issues/13549
    // to be fixed in https://github.com/OpenAPITools/openapi-generator/pull/13550
    importMapping().put("LinkedHashSet", importMapping().get("ArrayList"));

Looks like a bug of assuming "set" fields are instantiated with LinkedHashSet,
let's fix it by using type mappings.

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[dmivankov]

tagging Java	@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

[wing328]

thanks for the PR. can you please add some tests in https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/AbstractJavaCodegenTest.java to cover this change?

[dmivankov]

Added a test that was failing without this change

[wing328]

when you've time, can you please resolve the merge conflicts and PM me via Slack when it's done?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[dmivankov]

Mechanically resolved the conflict but didn't try to retest locally, will need to remember more context first :)