ssh: cli: Add support for uploading multiple keys to target

[posborne]

Currently, the algorithm for wva ssh authorize is the following:

1. Grab public key from current development machine
2. Write a authorized keys file to the WVA with the public key for the development machine

If there are multiple developers using a single device, this can be annoying as each developer needs to keep re-authorizing. The proposed change would be to do the following:

1. Grab the authorized keys file from the target if it exists
2. Add the development machine public key to the list of authorized keys (maybe do a set operation to avoid duplicates)
3. Write the modified authorized keys file back to the device