[Gradle Plugin] Allow to set additional property of any type. Useful for Boolean values.

[xsveda]

Gradle Plugin's additionalProperties are of type Map<String, String>. When a project tries to set a Boolean property it needs to put it as a String (i.e. "true") which is not evaluated as proper Boolean value by mustache template.

This PR update additionalProperties to Map<String, Any>.

Use case comes from moko-network library: icerockdev/moko-network#59

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

cc @OpenAPITools/generator-core-team

[wing328]

LGTM

[wing328]

@xsveda thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423