feat(prospective-parachains): handle ActiveLeaves signal

[EclesioMeloJunior]

Description

• Implement the View, which manages the subsystem inner state, no need for implementing Fragment Tree, I would suggest to create an interface to constraint Fragment tree behavior and use it for implementing this issue.
• Support metrics updates (use prometheus)

Routines needed:

• prospective_parachains_mode
• fetch_backing_state: this routine might be block due to the missing Runtime API subsystem, feat(dot/parachain): create RuntimeAPI subsystem #3940
• fetch_upcoming_paras
• fetch_block_info
• fetch_ancestry