add manual scaling to CLI

[iandouglas]

Customer request, but adding here so others can upvote.

Since CLI is built atop Render API, and the API has the means to scale a service up or down, it would be nice to support this in the CLI tool as well.

perhaps:
render services scale --id srv-abc123 --add 2
to add 2 more instances

and

render services scale --id srv-abc123 --remove 3
to remove 3 instances, if possible, maybe leaving one at a minimum that needs to be manually deleted to fully close down?