Create Rococo <> Westend bridge relayer in polkadot-sdk

[serban300]

Related to #2950

Right now the CI tests that we have inside polkadot-sdk use the runtimes from polkadot-sdk, and the relayer release from parity-bridges-common. The problem with this approach is that when we make a change to the relayer, the CI doesn't actually test it on the spot. For this kind of changes we have to run the tests locally manually.

We should create a Rococo <> Westend relayer flavor inside polkadot-sdk in order to address this limitation. For convenience we can also keep the Rococo <> Westend flavor inside the parity-bridges-common.