fix(ws): support custom bootstrap HTTP client

[Zhang-986]

Fixes #201.

Problem:
The websocket client fetched its connection URL through a package-level HTTP client. In restricted network or proxy environments, callers could not route that bootstrap request through their own *http.Client without changing global state.

Root cause:
getConnURL called the package-level bootstrapHTTPClient directly instead of using client instance configuration.

Change:

• Add WithHTTPClient(*http.Client) for the websocket client.
• Store the bootstrap HTTP client on Client.
• Keep the existing default behavior by initializing it from bootstrapHTTPClient.
• Use the configured client when fetching the websocket connection URL.

Scope:
This PR only addresses the HTTP request used to fetch the websocket connection URL. It does not change the websocket dialer behavior.

Verification:

• go test ./ws
• go test ./...