[v7] Add tsh config helper to generate OpenSSH client configuration (#7437)

[timothyb89]

• Add tsh config ssh helper to generate OpenSSH client configuration

This adds a new subcommand, tsh config ssh, to generate OpenSSH
client configuration snippets that allow users to connect directly to
nodes using the standard ssh client.

To support this change, tsh's known_hosts file has been modified to
match the format required by OpenSSH when verifying hosts against
certificates. Old-style known_hosts entries will be automatically
replaced and pruned when the end user first logs in with an updated
tsh. Small changes were additionally made to the keystore and key
agent to pass the proxy host into AddKnownHostKeys and to support
wildcard hostnames in known_hosts entries.

• Fix broken link to Trusted Clusters documentation

• Use text/template for SSH config generation; wrap all errors.

• Rename config helper from config ssh to just config

This changes the config helper to use just tsh config per
suggestion from @r0mant.

• Fix known_hosts_migrate_test after rebase

• First pass at review feedback

• Update docs/pages/server-access/guides/openssh.mdx

Co-authored-by: Roman Tkachenko [email protected]

• Ensure top-level hostnames never match wildcard patterns

• Add additional host count check to canPruneOldHostsEntry.

• Replace excess call to isOldStyleHostsEntry with documented invariant

• Trim trailing dots on absolute hostnames in matchesWildcard

Co-authored-by: Roman Tkachenko [email protected]

[russjones]

Bot.