FR: Ability to have segments that can be shared across projects

**Is your feature request related to a problem? Please describe.**

To have a separation of concern and to reduce the Flagsmith response size, We have created multiple Flagsmith projects that are being used by Mobile, FE, and BE teams. These projects usually don't have a common flag.

However, there are cases when a flag & its segments are needed in all projects. Currently, we go to app.flagsmith.com and manually create/update/duplicate segments for every project for this scenario but this is error-prone. A developer might miss updating segments in one of the projects and it could break everything.

**Describe the solution you'd like**
We were looking for a way in which we can have a shareable segment that could get used by all the projects, effortlessly.

**Describe alternatives you've considered**
One hack could be to create a common project and several individual projects and make at-least 2 flagsmith API calls to fetch flags and combine them but this is not suitable for our case. Neither flagsmith SDKs support that. So other than writing custom code, we don't have many options.

gz#439