Custom PR description message for canary releases

**Is your feature request related to a problem? Please describe.**

I would like to be able to customise the code block in the message that auto drops in a PR after a successful canary publish to NPM. I'm sure at some point in time I found documentation about it but looking at the code I don't think its possible. I tried doing the following in `.autorc`

```json5
"canary": {
"target": "pr-body",
"message": "Install PR version: `yarn add -D my-project@%v`",
"force": true // <-- just to see the results in the PR
},
```
But it only allows editing the summary not the code block.

**Describe the solution you'd like**

I would like to be able to customise the canary message in PR descriptions to read something like:

:sparkles: Test out this PR locally via:

```bash
npx @grafana/create-plugin@1.3.0-canary.226.9023d7b.0
# or
yarn create @grafana/plugin@1.3.0-canary.226.9023d7b.0
# or
pnpm dlx @grafana/plugin@1.3.0-canary.226.9023d7b.0
```

**Describe alternatives you've considered**

Not thought of any alternatives currently.

**Additional context**