Add support for asymmetric keys to the transit client

[MattDavis00]

Currently the read and export functions for the transit client do not support asymmetric keypairs.

The read response fails to deserialize, as the response differs for symmetric vs asymmetric keys.

Asymmetric read response:

// vault@926ee8c21172:/$ curl -XGET --insecure --silent -H "X-Vault-Token: $VAULT_TOKEN" $VAULT_ADDR/v1/transit/keys/my-new-key
{
    "request_id": "b59d5612-b16f-8742-641c-32e29943433f",
    "lease_id": "",
    "renewable": false,
    "lease_duration": 0,
    "data": {
        "allow_plaintext_backup": false,
        "auto_rotate_period": 0,
        "deletion_allowed": false,
        "derived": false,
        "exportable": false,
        "imported_key": false,
        "keys": {
            "1": {
                "creation_time": "2024-05-10T13:36:49.809157592Z",
                "name": "rsa-2048",
                "public_key": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA01QgwZl/tD7cWZl1NPQ2\nG8+cvI6KkF+wB94xfatdKJ07Ga0bsqYaeE98ZUZU7amRH9Kmdz/4Cu9xXdwlosSS\ntedXfRT8zcWpqEUpV4c4/lT/ox6W67KW2l44liiVt2f9PmviRoYclNWUJa6X+ZCU\n68p/Rd9RSb/xBsywTb8uJN1Q1cZM2JbKhWnaLa7jPsXG9hvnxJ2QFuN3+iZ2OCff\nCjhl0Anumc7lL6wEU/Yd1WuX+5afcHaGttHkWAasrhq3KFHhXlwf4+jrJ5C+aOfb\nevJgGUYjXgf4buV3sPFVmnhvyiyyEV/CysNbXx3KrI/OGgf7HQ+f3+hnpomo8lhd\nwwIDAQAB\n-----END PUBLIC KEY-----\n"
            },
            "2": {
                "creation_time": "2024-05-10T13:36:49.878041842Z",
                "name": "rsa-2048",
                "public_key": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA7Ttq3kD0Tev7uWnupVGS\neGl4IURKqiUpoPKdevpnYTfuKt/MVP9Ej1Qz7XOP3q6mC3Vb0mSK+6FNjWNakd7D\nQecJwVFuZXp4lF8uzTF5xXFiPiiw5WMS7v7oNmzTE15Msse1yLHcxQGveeRmrl1L\nTYzcLHb705AzgeXoHqLNNWfTC72fqlPhMvOIcWyWOatz/Dp9ukfcR7HAmxhlpluY\n8e+lift8xah4uQFFDuIwXBQ7f5G8+j9RKMyWjA8pv/pY12jDl/vOi6V2b5YJvJie\nVV9gNa1+9JjamG1bsmHYhRIK0rBTewdvD3Tyg+T8Yl5HQNqkIvdwaBZ/RIc+PVeI\ndwIDAQAB\n-----END PUBLIC KEY-----\n"
            },
            "3": {
                "creation_time": "2024-05-10T13:36:49.958724342Z",
                "name": "rsa-2048",
                "public_key": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAu5FQYHEs8U1QsnkTMDrB\n6MfpGK1eVdS84FepYyvpA//suDbJGS9l6XP333wuebwoTL42B5qiZ1tudbIeOWaK\nBDQ5dfjbjRB87k/h4nuQ2DNEUwhgVPjxS3XEocefVWxCfmwQqTjSvhVX2l3DmWiE\n1b/vgikBcYxkqTDTUytuN+IbvZmkFzcM+20UaemRQ5VeNP2MEjZsX4cBATFMkC6k\nYoXb3r+SeKd5JDp/qCSUeabkaxBoV1MprXCy8sEgxSEn/1SGOlVQvB9COQQ/Y/bq\nHQUNGKgrQWGXh4W+Uz4C/GIsKosVuTcGV/o/eONg5uft958NTa1vf93XaKpLOlbw\nOQIDAQAB\n-----END PUBLIC KEY-----\n"
            }
        },
        "latest_version": 3,
        "min_available_version": 0,
        "min_decryption_version": 1,
        "min_encryption_version": 0,
        "name": "my-new-key",
        "supports_decryption": true,
        "supports_derivation": false,
        "supports_encryption": true,
        "supports_signing": true,
        "type": "rsa-2048"
    },
    "wrap_info": null,
    "warnings": null,
    "auth": null
}

This does not follow the "{key_id}": {unix_timestamp} format that symmetric keys use, and hence does not deserialize.

A similar issue for the export route exists, where the only supported key types are:

pub enum ExportKeyType {
    EncryptionKey,
    SigningKey,
    HmacKey,
}

Where PublicKey is omitted.

It would be nice to support asymmetric keys; I will probably start working on some basic support in a fork, and see how I get along.