[WIP] Fix error when LB Rule action contains both target_group_arn and forward

[gdavison]

Fixes error when LB Rule action contains both target_group_arn and forward. While specifying both values is technically valid in the API, the logic in the provider ignores forward if target_group_arn is set.

This PR will disallow setting both.

Currently implemented for aws_lb_listener, but contains errors in plan-time validation. Needs to also be implemented for aws_lb_listener_rule

Relations

Closes #22526

References

Output from Acceptance Testing

% make testacc TESTS=TestAccXXX PKG=ec2

...

[github-actions]

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[gdavison]

Superseded by #35671

[github-actions]

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