cli: Add shell command

[armaniferrante]

Add a new anchor shell command that runs a node shell with generated clients ready to go, configured from the Anchor toml, for example,

cluster = "mainnet"
wallet = "<wallet>"

[clusters.mainnet]
lockup = "GrAkKfEpTKQuVHG2Y97Y2FF4i7y7Q5AHLK94JBy7Y5yv"
registry = "6ebQNeTPZ1j7k3TtkCCtEPRvG7GQsucQrZ7sSEDQi9Ks"

[clusters.devnet]
lockup = "<address-here>"
registry = "<address-here>"

Would allow one to run anchor shell to run a shell and communicate with the programs via anchor.workspace.<program-name> on mainnet. Or anchor shell --cluster devnet to do the same, but on devnet.