Support local actions defined in repository

**Is your feature request related to a problem? Please describe.**
I have a few locally created actions in the `.github/actions` directory but validation and autocomplete don't seem to work on these actions.

I have the following yaml file at `.github/actions/wait-for-deployment-action`

```yaml
name: Wait for deployment
description: |
Block until a deployment is finished for the pushed commit,
and get the URL for running any further tests.
branding:
icon: clock
color: black

inputs:
github-token:
description: Your GitHub access token
required: true
environment:
description: The deployment environment to wait for
timeout:
description: The number of seconds after which to fail if no deployment is found.
default: 30
interval:
description: The number of seconds to wait between repeated checks for deployments.
default: 5
sha:
description: |
The commit SHA to wait for. Defaults to the current commit SHA. For PRs, this is the merge commit SHA.
So provide ${{ github.event.pull_request.head.sha }} instead.

outputs:
url:
description: The deployment URL
id:
description: The GitHub deployment id

runs:
using: node16
main: dist/index.js
```

The below shows an attempt to autocomplete the params expected by a local action:

**Describe the solution you'd like**
For local actions to be validated and provide autocomplete for locally defined actions.

Start by locating the extension entry points for workflow validation and action-parameter autocomplete, then compare their handling of the local action metadata at `.github/actions/wait-for-deployment-action`. Use the shown action YAML, including its inputs and outputs, as the validation and completion case; done means local actions provide both without losing existing behavior.

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