Support subdomain isolation for GHES hosts

[SamMorrowDrums]

Right now we take a GHES hostname and use same domain, but actually subdomain isolation can be optionally enabled, and then for example https://{domain}/raw becomes https://raw.{domain}

func newGHESHost(hostname string) (apiHost, error) {
	u, err := url.Parse(hostname)
	if err != nil {
		return apiHost{}, fmt.Errorf("failed to parse GHES URL: %w", err)
	}

	restURL, err := url.Parse(fmt.Sprintf("%s://%s/api/v3/", u.Scheme, u.Hostname()))
	if err != nil {
		return apiHost{}, fmt.Errorf("failed to parse GHES REST URL: %w", err)
	}

	gqlURL, err := url.Parse(fmt.Sprintf("%s://%s/api/graphql", u.Scheme, u.Hostname()))
	if err != nil {
		return apiHost{}, fmt.Errorf("failed to parse GHES GraphQL URL: %w", err)
	}

	uploadURL, err := url.Parse(fmt.Sprintf("%s://%s/api/uploads/", u.Scheme, u.Hostname()))
	if err != nil {
		return apiHost{}, fmt.Errorf("failed to parse GHES Upload URL: %w", err)
	}
	rawURL, err := url.Parse(fmt.Sprintf("%s://%s/raw/", u.Scheme, u.Hostname()))
	if err != nil {
		return apiHost{}, fmt.Errorf("failed to parse GHES Raw URL: %w", err)
	}

	return apiHost{
		baseRESTURL: restURL,
		graphqlURL:  gqlURL,
		uploadURL:   uploadURL,
		rawURL:      rawURL,
	}, nil
}

[SamMorrowDrums]

👋 We're actively looking at improving GHES support for the MCP server! If you have thoughts on GHES-specific needs, please share your feedback in this discussion. 🙏