Vault Github App Credential

### What feature do you want to see added?

A new credential type: `Vault Github App Credential` similar to `Vault Github Token Creditential` and `GitHub App Credential`.

The GitHub plugin has a "Github App" credential type, that is used in github Jenkins orgranization folders and jobs. It has these fields (see image):

- `ID`
- `Description`
- `App ID`
- `Key`
- `Repository access strategy` -> `Owner` `Repositories` `Default permissions strategy`
- `GitHub organization to test against`

...of which only `Key` is a secret.

Seeing the Vault Plugin already has a `Vault Github Token Credentials`, I'd like to see a `Vault Github App Credential` that can be used in GitHub organizations and jobs. It would store the private key in Vault and otherwise behave like `GitHup App Credential`. 

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## A few screenshots, so it's clearer what I mean

### GitHub App Credential (already implemented - no Vault support)

<img width="1595" height="1168" alt="Image" src="https://github.com/user-attachments/assets/6c68866f-da52-4de2-943f-dda5823b74e4" />

### Vault GitHub App Credential (already implemented - has Vault support)

<img width="315" height="454" alt="Image" src="https://github.com/user-attachments/assets/d6d8c61f-e7b5-4e17-9dcc-5becd2887f68" />

### Organization Folder using a GitHub App Credential

<img width="635" height="646" alt="Image" src="https://github.com/user-attachments/assets/61a9aad8-74df-4473-a969-379bf4b4f769" />

### Creating a GitHub Organization Folder 

<img width="461" height="62" alt="Image" src="https://github.com/user-attachments/assets/a90874fd-012c-43e2-8e02-7ece6560f4b9" />

<img width="520" height="568" alt="Image" src="https://github.com/user-attachments/assets/a3304572-7561-4360-ae3c-bcdd697456d3" />

### Upstream changes

None I'd know of.

### Are you interested in contributing this feature?

I don't know Java or the Jenkins Plugin development, so no, it would be prohibitively expensive to me.

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Uh oh!

Vault Github App Credential #366

What feature do you want to see added?

A few screenshots, so it's clearer what I mean

GitHub App Credential (already implemented - no Vault support)