Implement blobs related Store methods

[StefanBratanov]

As part of implementing the P2P networking, some of the Store methods were left unimplemented in StoreTransaction, TestStoreImpl, CombinedChainDataClient, Store:

• getEarliestAvailableBlobsSidecarEpoch();
• getBlobsSidecarBySlotAndBlockRoot(final UInt64 slot, final Bytes32 blockRoot);
• retrieveSignedBlockAndBlobsSidecar(final Bytes32 blockRoot);