Handle multiple network endpoints and choose a healthy one with offchain indexing enabled

[clangenb]

As opposed to #1603, I did not introduce a connectivity store here. The current approach is quite simple in handling multiple endpoints and could easily be integrated in the current state of the app, otherwise we need more refactoring. Hence, I think that #1603 can still be closed until we realize that we need a more sophisticated solution. Closes #1603.

Changes:

• Introduces generic EndpointManager that has a list of endpoints and is generic over a mixin that checks the healthiness of the endpoints. With this, we can easily re-use the EndpointManager to handle multiple endpoints for ipfs and feed too.
• Upon app startup, we connect to a random endpoint of the hard-coded set if it is healthy - where healthy means that it is online and has offchain-indexing enabled (I had to realize that dwellir does not have offchain indexing enabled). Once our nodes are updated to polkadot-v1.14.0, we easily integrate this healthcheck too.
• During app operation, we regularly poll if the endpoint is still reachable and will reconnect to another if the connection is lost.

Future work:

• Use aggregated information from all endpoints to detect a network stall: Better Alert Message for Stalling Blockchain not Transaction Priority Error. #1471

Tests

• Tested locally that the integration tests work