roachprod: Add the ability to scp between roachprod instances

**Is your feature request related to a problem? Please describe.**
We need to be able to scp files from one roachprod instance to another. This will help avoid uploading very large files from local machines to roachprod instances (e.g. tsdump.gob).

**Describe the solution you'd like**
SCP between roachprod instances, e.g.:

`rochprod scp INSTANCE1(source) INSTANCE2(destination) filename`

**Describe alternatives you've considered**
It is currently possible to just use scp from one machine to another, but certificates make it tricky. Example process:

1. ssh into destination instance.
2. copy private key
3. ssh into source instance
4. create key from copied key
5. run scp -i pointing to new key, e.g.
`scp -i ~/.ssh/instance2_key /mnt/data1/example.txt X.X.X.X:/mnt/data1/example.txt`

Jira issue: CRDB-29138