feat: Nexus extension

[boolivar]

NexusExtension with ANALYZER_NEXUS_USER and ANALYZER_NEXUS_PASSWORD settings for dependencyCheck tasks.

Also AnalyzerExtension updated to Gradle managed for proper DSL support i.e.

dependencyCheck {
    analyzers {
        nexus {
            enabled = true
            url = "https://nexus.local"
            username = nexusUser
            password = nexusPass
        }
    }
}

[jeremylong]

Thanks for the PR!