Allow ssh_config generation to be disabled for Machine ID Identity Output

[strideynet]

We currently allow the Identity Output to be used to provide credentials for two main use-cases:

• Server Access (SSH)
• General API access (tctl, access plugins)

However, we always try to generate the ssh_config needed for SSH access. This is problematic for a few reasons:

• It requires a proxy to be online so that it can determine the address to include in the SSH config. In some cases, a tbot instance may run close to the auth server and it may be intended for this instance to come online before proxies are available.
• It's slower than necessary because of the time taken to generate the SSH Config.

As such, I've added an ssh_config option which can be set to on or off. If unspecified, it defaults to the behaviour we've had historically (on).

example:

- type: identity
  destination:
    type: directory
    path: /foo
  ssh_config: off # this will default to "on" if unspecified

changelog: Add the ability to control ssh_config generation in Machine ID's Identity Outputs. This allows the generation of the ssh_config to be disabled if unnecessary, improving performance and removing the dependency on the Proxy being online.

[public-teleport-github-review-bot]

@strideynet See the table below for backport results.

Branch	Result
branch/v14	Create PR
branch/v15	Create PR