[FEAT]: issue_labels->color: support # as prefix to visualize the color in IDE's

### Describe the need

Hi everyone,
the `issue_label` resource lets you define a color for labels.

You'll need a six-digit HEX code without the `#` as prefix. The downside is that the `#` is used, for example, in VSCode to visually display the color.

It would be great if the provider supported the `#` prefix and simply stripped it out internally so that users can see the color in the IDE.

Here are two screenshots:

### GitHub Installation Type

- [x] GitHub.com (Free, Pro, or Team)
- [ ] GitHub Enterprise Server (on-premises)
- [ ] GitHub Enterprise Cloud with Personal Accounts (github.com)
- [ ] GitHub Enterprise Cloud with Managed Users/EMU (github.com)
- [ ] GitHub Enterprise Cloud with Data Residency (*.ghe.com)
- [ ] I don't know

### Relevant log output

```shell

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start by locating the issue_label resource and its existing validation and tests. Check how the color is read during configuration and written to GitHub, then add coverage showing that a six-digit color with a # prefix is accepted while the stored/API value remains without the prefix.

Written by the indexing model from the issue text.