Configure Script Approvals

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I don't think there are any dependencies to implement this, but I don't know enough about the internals to know for sure.

### Feature Request

It is already possible to configure approvedSignatures:

```
security:
scriptApproval:
approvedSignatures:
- "some signature"
```

But it isn't possible to configure approved scripts or hashes of approved scripts, which is what's technically being approved.

I'm working on migrating from [jenkins-startup-scripts](https://github.com/buildit/jenkins-startup-scripts), where this is a built in feature. The [implementation](https://github.com/buildit/jenkins-startup-scripts/blob/master/scripts/scripts/scriptsecurity.groovy) is quite simple, so I don't see why this couldn't be included in JCasC.

Obtaining the hashes is a bit of a pain, as it requires you to run the job that needs the script approvals, then approve them in the UI and finally grab the hashes from `scriptApproval.xml` in `JENKINS_HOME`. So we might want to simplify that by letting the user provide the whole script, but getting feature parity with the [jenkins-startup-scripts](https://github.com/buildit/jenkins-startup-scripts) is a good start.

So here is my suggestion:

```
security:
scriptApproval:
approvedScriptHashes:
- "some hash"
```

To make life easier, we could also have:

```
security:
scriptApproval:
approvedScripts:
- "some script"
```

However that is a bit more effort and mostly a nice to have. The first one is essential, though.

I'm happy to create a PR, but I don't have any experience with this code base, so I would be happy if someone wants to help out. Also, if there are any workarounds available right now, please let me know.