Allow specifying the task to be run

[mridang]

The current approach introspects the Gradle tasks to understand what should be run. This is a bit verbose. It would be simpler if we could just specify the name of the task e.g.

{
  "branches": [
    "master"
  ],
  "plugins": [
    "@semantic-release/commit-analyzer",
    "@semantic-release/release-notes-generator",
    [
      "@semantic-release/exec",
      {
        "prepareCmd": "./gradlew build",
        "publishCmd": "./gradlew publish"
      }
    ],
    [
      "gradle-semantic-release-plugin",
      {
        "releaseCmd": "./gradlew publishPlugins"
      }
    ],
    [
      "@semantic-release/github",
      {
        "assets": [
          
        ]
      }
    ],
    [
      "@semantic-release/git",
      {
        "message": "chore(release): ${nextRelease.version} [skip ci]\n\n${nextRelease.notes}",
        "assets": [
            "gradle.properties"
          ]
      }
    ]
  ]
}