confluence-task: support for workflow default variable

[benbroadaway]

Currently, the runtime-v2 confluence plugin does not support workflow-defined default variables like the documentation suggests.

configuration:
  arguments:
    confluenceParams:
      apiUrl: "https://confluence.example.com/rest/api/"

It does support policy-defined default variables, but a user can't use a "global" workflow variable defined in configuration.arguments to override the defaults.

concord-plugins/tasks/confluence/src/main/java/com/walmartlabs/concord/plugins/confluence/v2/ConfluenceTaskV2.java

Lines 46 to 47 in 1dd5cae

	Result result = new ConfluenceTaskCommon(context.workingDirectory(), context.variables().toMap())
	.execute(TaskParams.of(input, context.defaultVariables().toMap()));

For parity with the runtime-v1 task implementation, it should look for a similar default global variable and let it override the policy-defined default.