Add option to load accounts from file

This introduces the `--clone-from-file` option for
solana-test-validator. It allows specifying any number of files
(without extension) containing account info and data, which will be
loaded at genesis. This is similar to `--bpf-program` for programs
loading.

The files will be searched for in the CWD or in `tests/fixtures`.

Example: `solana-test-validator --clone-from-file SRM_token USD_token`

docs/src/developing/test-validator.md

@@ -14,6 +14,7 @@ starts a full-featured, single-node cluster on the developer's workstation.
 - Direct [on-chain program](on-chain-programs/overview) deployment
   (`--bpf-program ...`)
 - Clone accounts from a public cluster, including programs (`--clone ...`)
+- Load accounts from files
 - Configurable transaction history retention (`--limit-ledger-size ...`)
 - Configurable epoch length (`--slots-per-epoch ...`)
 - Jump to an arbitrary slot (`--warp-slot ...`)