Add `.Complete` to Enhanced Depends

# Summary

Currently using the [directed-acyclic graph](https://argoproj.github.io/argo/examples/#dag) workflow template and the [enhanced depends logic](https://argoproj.github.io/argo/enhanced-depends-logic/) it's possible to declare that a step should run if a specific previous step failed.

To give a simple example, here if A is successful then B will run. If A fails then C will run instead.
```yaml
- name: A
- name: B
depends: "A"
- name: C
depends: "A.Failed"
```

Along with the logical or (`||`) operator, it is possible to declare a step should run after another step, irrespective of if that step succeeds or fails.
```yaml
- name: A
- name: B
depends: "A || A.Failed"
```

However this only works if A was run at all. If instead it is at the bottom of a longer chain of dependencies with a failure higher up then it is not possible to execute B.

For example
```yaml
- name: A
- name: B
depends: "A"
- name: C
depends: "B"
- name: D
depends: "C || C.Failed"
```

In this chan if C fails then D will run, but if A or B fail then D will not run. The only way to specify that D should always run, but only after C is to set D's `depends` to `C || C.Failed || B.Failed || A.Failed`. This is workable but becomes difficult to maintain if there is a long chain of steps with a "always run" step at the bottom, and critically is easy to miss a step.

Changing existing behaviour is likely a bad idea, but a new syntax to support this case could work. For example `step.Complete` could mean "step completed in any way. Including not run at all." but other language may be less confusing.

# Use Cases

A common use case is to perform some cleanup after a series of steps. To give a more complete real-world example:

```yaml
- name: download-terraform
- name: create-test-environment
depends: "download-terraform"
arguments:
artifacts:
- name: terraform
from: '{{tasks.download-terraform.outputs.artifacts.terraform}}'
- name: configure-test-environment
depends: "create-test-environment"
- name: install-project
depends: "configure-test-environment"
- name: run-e2e-tests
depends: "install-project"
- name: destroy-test-environment
depends: "create-test-environment && (run-e2e-tests || run-e2e-tests.Failed || install-project.Failed || configure-test-environment.Failed)"
arguments:
artifacts:
- name: terraform
from: '{{tasks.download-terraform.outputs.artifacts.terraform}}'
```

Here we create a test environment, perform a number of steps and ultimately run some e2e tests on our project. Then we want to destroy the test environment. But even if any of the intermediate environment configuration steps fail we want to still destroy the test environment, so long as the original `create-test-environment` step ran. So we'd like to simply that depends down from `create-test-environment && (run-e2e-tests || run-e2e-tests.Failed || install-project.Failed || configure-test-environment.Failed)` to something more like `create-test-environment && run-e2e-tests.Complete`.

---

**Message from the maintainers**:

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Start with the enhanced depends logic documentation and DAG examples linked in the issue, then trace the dependency evaluation entry point for `depends` expressions. Define and test the proposed completion semantics, including skipped ancestors and failures, and verify the cleanup example can use `create-test-environment && run-e2e-tests.Complete`.

Written by the indexing model from the issue text.