feat(kms): add key resource and datasource (#1055)

relates to STACKITTPR-411

docs/data-sources/kms_key.md

@@ -0,0 +1,45 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_kms_key Data Source - stackit"
+subcategory: ""
+description: |-
+  KMS Key datasource schema. Uses the default_region specified in the provider configuration as a fallback in case no region is defined on datasource level.
+---
+
+# stackit_kms_key (Data Source)
+
+KMS Key datasource schema. Uses the `default_region` specified in the provider configuration as a fallback in case no `region` is defined on datasource level.
+
+## Example Usage
+
+```terraform
+data "stackit_kms_key" "key" {
+  project_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  keyring_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  key_id     = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `key_id` (String) The ID of the key
+- `keyring_id` (String) The ID of the associated key ring
+- `project_id` (String) STACKIT project ID to which the key is associated.
+
+### Optional
+
+- `region` (String) The resource region. If not defined, the provider region is used.
+
+### Read-Only
+
+- `access_scope` (String) The access scope of the key. Default is `PUBLIC`. Possible values are: `PUBLIC`, `SNA`.
+- `algorithm` (String) The encryption algorithm that the key will use to encrypt data. Possible values are: `aes_256_gcm`, `rsa_2048_oaep_sha256`, `rsa_3072_oaep_sha256`, `rsa_4096_oaep_sha256`, `rsa_4096_oaep_sha512`, `hmac_sha256`, `hmac_sha384`, `hmac_sha512`, `ecdsa_p256_sha256`, `ecdsa_p384_sha384`, `ecdsa_p521_sha512`.
+- `description` (String) A user chosen description to distinguish multiple keys
+- `display_name` (String) The display name to distinguish multiple keys
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`region`,`keyring_id`,`key_id`".
+- `import_only` (Boolean) States whether versions can be created or only imported.
+- `protection` (String) The underlying system that is responsible for protecting the key material. Possible values are: `software`.
+- `purpose` (String) The purpose for which the key will be used. Possible values are: `symmetric_encrypt_decrypt`, `asymmetric_encrypt_decrypt`, `message_authentication_code`, `asymmetric_sign_verify`.