Allow specifying custom parameters

The library locks you into providing only specific parameters. For example, `\Api\CurrentUser`:

The `/user/repos` endpoint has more parameters than just these, though, such as `per_page`, which is now being locked to 30 because users cannot specify a custom value.

If there *is* however a way to specify custom options without having to do any hacky things with `$client->getHttpClient`, I'd love to hear.

Start by examining Api\CurrentUser and how requests to /user/repos accept and pass parameters; the issue specifically mentions the client’s getHttpClient as a possible workaround. Determine how custom options such as per_page should be exposed, then verify that callers can override the current fixed value without relying on the underlying HTTP client.

索引モデルが issue の本文から書いたものです。