support server-side flag to append ServerAliveInterval 5 to SSH entries made by Gateway

[ericpaulsen]

@code-asher resolved periodic disconnects on a Coder workspace connected from a JetBrains IDE by having the user set the following in their SSH config:

Host coder-jetbrains--*
  ServerAliveInterval 5

problem is, the SSH config will be overwritten by Gateway each time it authenticates to Coder. we should support a server flag to append ServerAliveInterval 5 to Gateway entries. we already support this for standard SSH connections via --ssh-config-options ServerAliveInterval=x

i confirmed the above flag does not apply to Gateway entries in the SSH config.