Gateway exports

[jvstme]

Allow exporting gateways to other projects.

$ dstack export --project main
 NAME       FLEETS   GATEWAYS      IMPORTERS 
 my-export  -        main-gateway  team      

$ dstack import --project team
 NAME            FLEETS   GATEWAYS     
 main/my-export  -        main-gateway 

$ dstack gateway --project team
 NAME               BACKEND          HOSTNAME        DOMAIN                 DEFAULT  STATUS  
 main/main-gateway  aws (eu-west-1)  108.131.126.35  gtw.mycompany.example           running

Imported gateways can be used by specifying them in the gateway property in service configurations.

type: service
port: 80
image: nginx
gateway: main/main-gateway

Limitations:

• Setting an imported gateway as the default gateway is not yet supported.
• Evicting services from a gateway that is no longer imported is not yet supported.

#3851

[jvstme]

@r4victor, sorry for the huge PR. It's mostly boilerplate, but at least server/services and server/security are worth reviewing

[r4victor]

It seems ProjectMemberOrPublicAccess() was used so that everyone could get gateways in public projects. Is the tightening intentional? Does the UI for public projects continue to work without problems?

[jvstme]

Wasn't intentional, thanks