Improve gradle build of the swan lake plugin

[NipunaRanasinghe]

Description: The gradle build of the Ballerina swan lake plugin should be improved to

• run plugin verifier as a part of the build
• include spotbugs as the static analysis tool for the java implmentation
• use optimized code with best practices

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[NipunaRanasinghe]

Closing with https://github.com/ballerina-platform/plugin-intellij/pull/59