feat(ssh): add support for `key_type` parameter

[radek-sprta]

Description

Add support for key_type parameter in SSH Secret Engine, so we can specify types other than the default ssh-rsa.

Checklist

• [x] Added CHANGELOG entry (only for user-facing changes)
• [x] Acceptance tests were run against all supported Vault Versions

Output from acceptance testing:

=== RUN   TestAccSSHSecretBackendCA_basic                                                              
--- PASS: TestAccSSHSecretBackendCA_basic (2.28s)                                                      
=== RUN   TestAccSSHSecretBackendCA_provided                                                                                                                                                                   
--- PASS: TestAccSSHSecretBackendCA_provided (1.68s)                                                                                                                                                           
=== RUN   TestAccSSHSecretBackend_import                                                               
--- PASS: TestAccSSHSecretBackend_import (2.66s)          
...

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