Add support for Server Sent Events (SSE)

[bachp]

This extension allows to handle SSE events
by invoking a callback whenever an event is received.

Related documentation: libcpr/docs#59

Closes #1014

I tested this against an axum server with the following example:

#include <cpr/cpr.h>
#include <iostream>

int main() {
    std::cout << "Connecting to SSE endpoint at http://localhost:9976/hello..." << std::endl;

    cpr::Session session;
    session.SetUrl(cpr::Url{"http://localhost:9976/hello"});

    // Set up SSE callback to print events to console
    session.SetServerSentEventCallback(
        cpr::ServerSentEventCallback{
            [](cpr::ServerSentEvent&& event, intptr_t userdata) {
                std::cout << "=== New Event ===" << std::endl;
                std::cout << "Event Type: " << event.event << std::endl;
                std::cout << "Data: " << event.data << std::endl;

                if (event.id.has_value()) {
                    std::cout << "ID: " << event.id.value() << std::endl;
                }

                if (event.retry.has_value()) {
                    std::cout << "Retry: " << event.retry.value() << " ms" << std::endl;
                }

                std::cout << std::endl;

                // Return true to continue receiving events
                return true;
            }
        }
    );

    cpr::Response response = session.Get();

    std::cout << "Connection closed." << std::endl;
    std::cout << "Status Code: " << response.status_code << std::endl;

    return 0;
}

[COM8]

@bachp Thank you very much for contributing this!
I only have a minor comment and clang-tidy is not yet completely happy. The abseil-string-find-str-contains clang-tidy error can be added to the ignore list inside .clang-tidy.
The other CI runs are currently expected to fail.

[bachp]

I fixed all the linter errors.

[COM8]

@bachp Please rabase and then we are ready to go.

[COM8]

@bachp I rebased and then created a new release: https://github.com/libcpr/cpr/releases/tag/1.14.0