Add pallets/approved-referenda endpoint

[Imod7]

Description

Related to #1431

Add a new endpoint pallets/approved-referenda to retrieve information of scheduled Approved referenda of (potential) runtime upgrades and similar referenda created for root and whitelisted caller tracks (track 0 and 1 respectively) of OpenGov.

Issue that it solves

With the existing endpoint pallets/on-going-referenda (#1471) we only show the referenda that are in status OnGoing so as soon as a Referendum gets Approved we stopped showing it. However from the time it gets Approved until its Enacted there is still quite a lot of block time.

Example

• 373 Upgrade Kusama v1.2.0 -> enactment at block #22790000 - code Updated event
  - block 22691622 : last block when ref373 is in status OnGoing -> so it is still shown by the pallets/on-going-referenda endpoint
  - block 22691623 : ref373 gets Approved -> so we stop showing it in the endpoint's response
  - block 22790000 : ref373 gets enacted

so during approx 98.000 blocks there is still a Runtime Upgrade coming but we do not show it.

Possible Issue
Someone checks the endpoint after the referenda of the runtime upgrade was approved so they miss it.

Additional Information

Idea from @IkerAlus
Add a small section in the endpoints doc for Governance, and consider including other additions in the future, such as additional tracks and other relevant content.

Thanks & Credits

@IkerAlus for all the valuable feedback on the idea of showing also approved referenda. Credits to him for suggesting a whole separate endpoint.