Fix SSH API to publish more things

SshKeyAccessService is an internal API and should be made public so that third-party add-ons can use it to manage SSH access keys and bring it inline with what the REST APIs can do.

KeyUtils is in the com.atlassian.stash.ssh.utils package which, lacking internal seems like it should be part of the API. However, the Export-Package is setup as com.atlassian.stash.ssh.api, so KeyUtils is not exported.