feat: expose http.Client override through the main configuration

[jqueuniet]

An override to the remote http.Client has been added to the remote configuration with PR #101 , but using it seems to require reimplementing the Start function from the api package (unless I'm missing something). This PR adds an HTTPClient field to the main configuration struct to facilitate customization.

I guess there could be many different reasons to justify this, mine is to allow overriding the CA certificate store without rebuilding the system CA store, which can be difficult or even impossible in specific contexts.

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