feat (@jitsu/js): Add defaultPayloadContext to include default context in all requests

[dmeremyanin]

This PR introduces a new property defaultPayloadContext to the configuration. It allows default context data (e.g., browser timezone, some extra user identifier) to be automatically included in every request's payload.

Example Usage

window.jitsuConfig = {
  defaultPayloadContext: {
    browserTz: Intl.DateTimeFormat().resolvedOptions().timeZone,
    userIdentity: getUserIdentity(),
  }
}

The values of browserTz and userIdentity will now be included in the payload of every request, ensuring that the common context data is consistently available in all API calls, without needing to manually include them with each request.

[dmeremyanin]

@absorbb thanks for the prompt review of all the PRs. Also, just wondering - when can we expect a new release with the latest changes?

[absorbb]

Usually we make a release to fix some critical issue or when we collected significant amount of minor changes.
But these changes are already in beta images. So you can switch to beta until we issue another release.

[dmeremyanin]

Got it, thanks for the clarification. We will switch to the beta images in the meantime.