[FEATURE REQUEST] Provide Vault SSH certificate backend for new `ssh_pki` state module

[lkubb]

Is your feature request related to a problem? Please describe.
Vault provides an SSH CA. Salt 3008 will introduce new ssh_pki modules, which work similar to the x509 ones. In contrast to the x509 modules (currently at least), the ssh_pki.certificate_managed state provides an option to use a custom backend for certificate creation (instead of the builtin ssh_pki.create_certificate execution module function): https://github.com/saltstack/salt/blob/7a8d343473ad826ccfbe7bc40dd2e76808e4f82f/salt/states/ssh_pki.py#L236-L239

Describe the solution you'd like
Make the vault_ssh execution module compatible to be used as the backend for the new ssh_pki.certificate_managed state.

Describe alternatives you've considered
None.

Additional context
The necessary code already exists: https://github.com/lkubb/salt-vault-formula/blob/6623f453c308569c543de665cc6f3ff3ffbfe0fa/_modules/vault_ssh.py#L900-L1089

Just need to add it here and write tests, but the tests require Salt 3008 to work. Consider testing against nightly or wait for the release of 3008.