[google_font] Move the httpClient from a package level variable to the Config object

[bramp]

In progress: I have not written changelog/documentation updates. I want to get early feedback / validation on this idea before I do the remaining work.

Move the httpClient from a package level variable, to be on the Config object

This allows the user to customise the httpClient as well as mock it out in tests.

Fixes #182429

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[gemini-code-assist]

Code Review

This pull request refactors the httpClient to be part of the GoogleFonts.config object. This is a good change that improves testability and allows users to provide a custom HTTP client. However, I've identified a potential resource leak issue with the default http.Client instance not being closed. My review includes a comment with details on this issue.

[gemini-code-assist]

The http.Client should be closed when it's no longer needed to release underlying resources. The current implementation creates a default client but never closes it, which can lead to resource leaks.

Please consider how the client's lifecycle will be managed. For example, you could add a static GoogleFonts.dispose() method that closes the default client. You should also document that if a user supplies their own httpClient, they are responsible for closing it.