New "capabilities" method

Had a discussion with @pipermerriam and he had a really good recommendation that there should be an interface that allows clients to communicate to users what they are "capable" of serving. A few example client responses:

> "I _only_ store the last N blocks. I can't tell you information about blocks before that."

> "I don't maintain a block number to block hash mapping, so you can only request information using the block hash."

> "I don't participate in the tx pool, so you'll need to send your transactions to a different client if you want them to propagate to the network."

With the merge on the horizon, this could be an important new method in execution client api as "finality" may cause users to finally start dropping old data. Without a way of communicating that, clients may return confusing information like "not found".