Support mTLS authentication with KMIP servers. (resolves #982)

[ximon18]

• Extend the Cascade CLI kmip add subcommand to take an optional SNI --server-name argument.
• Extend the Cascade CLI kmip show subcommand to report the SNI server name value that is configured.
• Extend the Cascade API HsmAddError type to also report missing client certificate or key, or I/O errors (if the server is unable to write the given certificate and key files to its filesystem).
• Improve the user experience of adding a KMIP HSM via the CLI with additional feedback about what is happening.
• Factor out Json type wrapping in the API kmip_server_add() function to a wrapper fn to improve readability.
• Extend the API kmip_server_add() function to write given certificate and key files to the server filesystem on connection success.
• Extend KMIP state file handling code to read and write mTLS related details.
• Extend the key manager unit to read mTLS related details from the KMIP state file and to pass mTLS related KMIP command line arguments to the dnst keyset kmip add-server subcommand.
• Factor KMIP credential file access out of the key manager unit to src/common/kmip_creds.rs.
• Rename KmpClientCredentialsFile to KmipCredentialsManager and add fn save_mtls_data() to save mTLS certificate files to disk.
• Factor KMIP mTLS certificate and key loading out of the zone signer unit into KmipCredentialsManager.
• Update signer key loading to use KmipCredentialsManager::load_mtls_data().
• Follow changed location in domain_kmip of export SyncConnPool type.

Note: Draft because it requires updated dnst, domain-kmip and kmip-protocol crates that haven't been released yet, and probably still needs more polishing and the new SNI server name command line argument still needs documenting.

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