Proxies are possible in the runtimes for Kusama and Polkadot but this
functionality was not previously available on testnets.

Closes #5453.

Proxies can now be used on `coretime-rococo`, `coretime-westend`,
`people-rococo` and `people-westend` in the same way as they can be on
Kusama and Polkadot chains.

The exact same proxies are configured as the production runtimes for the
respective system parachains.