feat: replay option for RxReq

[penpenpng]

resolve #161.

Added replay option to allow emit() before use(). This option will be enabled by default in the future.

const rxReq = createRxBackwardReq({ replay: true });

rxReq.emit({ ids: [ID_1] });
rxReq.emit({ ids: [ID_2] });
rxReq.emit({ ids: [ID_3] });

rxNostr.use(rxReq).subscribe(() => { /* ... */ });

// Note that it can be reused.
// rxNostr.use(rxReq).subscribe(() => { /* ... */ });