Output ExecutionPayload json to file

[Dan Cline]

newPayload jsons are often over the limit for bash and sh, which is 32 times the OS page size. This means that the default of execution-payload-builder, which outputs a cast command, sometimes will never work. cast supports std, so the correct way to deal with large payloads is to put the payload in a file, and do this:

cat ~/newpayload-123456.json | cast rpc --jwt-secret $JWT_SECRET engine_newPayloadV3 --raw

We should support this use case with a command line arg to specify file output like this. Note that the json should NOT have surrounding single quotes like the current output does.