feat(coderd): connect dbcrypt package implementation (#9523)

See also: https://github.com/coder/coder/pull/9522

- Adds commands `server dbcrypt {rotate,decrypt,delete}` to re-encrypt, decrypt, or delete encrypted data, respectively.
- Plumbs through dbcrypt in enterprise/coderd (including unit tests).
- Adds documentation in admin/encryption.md.

This enables dbcrypt by default, but the feature is soft-enforced on supplying external token encryption keys. Without specifying any keys, encryption/decryption is a no-op.

cli/testdata/coder_server_--help.golden

@@ -458,6 +458,16 @@ These options are only available in the Enterprise Edition.
           An HTTP URL that is accessible by other replicas to relay DERP
           traffic. Required for high availability.
 
+      --external-token-encryption-keys string-array, $CODER_EXTERNAL_TOKEN_ENCRYPTION_KEYS
+          Encrypt OIDC and Git authentication tokens with AES-256-GCM in the
+          database. The value must be a comma-separated list of base64-encoded
+          keys. Each key, when base64-decoded, must be exactly 32 bytes in
+          length. The first key will be used to encrypt new values. Subsequent
+          keys will be used as a fallback when decrypting. During normal
+          operation it is recommended to only set one key unless you are in the
+          process of rotating keys with the `coder server dbcrypt rotate`
+          command.
+
       --scim-auth-header string, $CODER_SCIM_AUTH_HEADER
           Enables SCIM and sets the authentication header for the built-in SCIM
           server. New users are automatically created with OIDC authentication.