feat: Add ability to POST an SSE request

[Gao-Jun]

Background

Recently, I'm working on an SSE client to Tencent LKE interface, which use a POST request and reply SSE AIGC content. ruby-eventsource is a good gem to handle SSE replies, but cannot send request via POST.

Solution

This PR add a new param payload to initialize method. Some behaviors will be changed if payload is set.

1. Request will be sent via HTTP POST method with payload content
2. Reconnection is disabled to avoid duplicated POST

[keelerm84]

Thank you for your interest and contribution. I do not have the capacity to review this PR quite yet, but I will get to it ASAP.

[aunghtain]

i need this feature as well. when will it get merged? Thanks