feat: Add WithHttpClient option to allow custom http.Client

[rv-ekroener]

Component(s)

router

Is your feature request related to a problem? Please describe.

The router-plugin/httpclient package does not expose a way to supply a custom *http.Client, despite the internal Client struct already having a client *http.Client field.

Currently, the New() function hardcodes http.DefaultClient:

c := &Client{
    client: http.DefaultClient,
    // ...
}

This forces developers to modify http.DefaultClient directly for use cases like disabling redirects:

// Current workaround: modifying global default client
http.DefaultClient = &http.Client{
    CheckRedirect: func(req *http.Request, via []*http.Request) error {
        return http.ErrUseLastResponse
    },
}

client := httpclient.New(
    httpclient.WithBaseURL(baseURL),
    // ...
)

This is problematic because:

1. It affects all HTTP clients in the process globally
2. It's not thread-safe
3. WithTimeout() also mutates http.DefaultClient.Timeout, compounding the issue

Describe the solution you'd like

Add a WithHttpClient option:

func WithHttpClient(client *http.Client) ClientOption {
    return func(c *Client) {
        c.client = client
    }
}

Usage:

customClient := &http.Client{
    CheckRedirect: func(req *http.Request, via []*http.Request) error {
        return http.ErrUseLastResponse
    },
    Transport: myCustomTransport,
    Timeout:   10 * time.Second,
}

client := httpclient.New(
    httpclient.WithBaseURL("https://api.example.com"),
    httpclient.WithHttpClient(customClient),
)

Describe alternatives you've considered

1. Modifying http.DefaultClient globally — Current workaround. Affects all HTTP clients, not thread-safe.

2. Adding individual options (e.g., WithDisableRedirects, WithTransport) — Less flexible than a single BYO client option.

Additional context

Use cases this would enable:

• Disabling redirects — Required when APIs return data via redirect headers (e.g., Location on 302)
• HTTP request/response logging — Inject a logging RoundTripper
• Custom TLS configuration — Client certificates, custom CA pools
• Custom connection pooling — Tune MaxIdleConns, IdleConnTimeout
• Proxy configuration — Per-client proxy settings
• Testing — Inject mock transports for unit tests

Implementation notes:

• The Client struct already has the client *http.Client field — this is just exposing it
• Non-breaking change — existing behavior unchanged if option is not used
• Consider: if WithHttpClient is used alongside WithTimeout, should timeout be applied to the custom client or ignored? (Suggest: ignore, let the user control their own client fully)

Prior art:

• resty.SetClient()
• retryablehttp.Client.HTTPClient
• Standard Go pattern for HTTP client libraries

[github-actions]

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