Add per-task `skipCheckerFramework` option.

[9999years]

• Introduces CheckerFrameworkTaskExtension for per-task checker configuration.
• Currently, the only option is skipCheckerFramework.

Partially fixes #110 (task-dependent configuration) and #106 (skip checks for autogenerated code).

We can use this to skip checking code generated by rest.li's Pegasus compiler:

subprojects {
  plugins.withType(JavaPlugin) {
    project.apply plugin: 'org.checkerframework'
    tasks.withType(JavaCompile).configureEach {
      // Don't run the checker on generated code.
      if (name.equals("compileMainGeneratedDataTemplateJava")
          || name.equals("compileMainGeneratedRestJava")) {
        checkerFramework {
          skipCheckerFramework = true
        }
      }
    }
  }
}

[mernst]

@9999years Nice enhancement, thanks.