Add option for HotShot to not fetch transactions (or to only fetch from certain networks)

[elliedavidson]

If a node wishes to only ever use a builder service, it should be able to configure HotShot to never fetch transactions, since it has no need for them. We should also configure HotShot to choose to only fetch transactions from the CDN or libp2p, if node operators want more flexibility. (For example they are running a builder that fetches from the CDN already).