SSE: Make HTTP Method configurable

[jeevatkm]

Discussed in #979

^{Originally posted by iyaozhen March 6, 2025}

func (es *EventSource) createRequest() (*http.Request, error) {
	req, err := http.NewRequest(MethodGet, es.url, nil)
	if err != nil {
		return nil, err
	}

Normally, the SSE protocol only supports GET. But some server require POST json.

[jeevatkm]

Implemented by PR #988