gitlab supportedDomain modifies the domain name and returns an url

[enekofb]

Describe the bug

Gitlab SupportedDomain function implementation
returns a URL which seems to be breaking the SupportedDomain contract

It also updates the field overwriting the original value

To Reproduce

Create a test case like the following

	clientOptions := []gitprovider.ClientOption{}

	clientOptions = append(clientOptions, gitprovider.WithDomain("gitlab.git.xx"))

	c, err := gitlab.NewClient(os.Getenv("GITLAB_ENTERPRISE_TOKEN"), "", clientOptions...)
	checkErr(err)
	log.Printf("client supported domain: %s", c.SupportedDomain())

it will output a domain name value which is an URL

2022/12/28 22:44:08 client supported domain: https://gitlab.git.xx```

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Steps to reproduce the behaviour
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### Expected behavior

A clear and concise description of what you expected to happen.

Given the previous example, I would expect to return a domain name instead of a url

```bash
2022/12/28 22:44:08 client supported domain: gitlab.git.xx```

### Additional context

- Go version: go version go1.19.1 darwin/arm64
- Git provider: gitlab