Adr for kine support

Signed-off-by: Vitor Savian <[email protected]>

docs/adrs/007-add-kine-support.md

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+# Add kine support to RKE2
+
+## Established
+
+## Revisit by
+
+## Status
+
+Proposed
+
+## Context
+
+This ADR is a introduction of kine support for RKE2. However, for this support to be implemented, it was necessary to add kine with TLS in K3s.
+Which was done in this [PR](https://github.com/k3s-io/k3s/pull/9572), It was needed since rke2 cannot connect to kine without tls via the api server.
+
+When rke2 is started with the `--datastore-endpoint` flag, it will disable the etcd pod and set the `cluster-init` flag to be `false`, to avoid the etcd part of k3s to be started.
+Kine will use the etcd client certificate to authenticate the connection to the kine server that will be a `unixs` socket type.
+
+### Pros
+
+- With the integration of kine, it is now possible to use the `--datastore-endpoint` flag among others related to kine. This allows for a more versatile configuration of the datastore,
+providing users with the flexibility to choose their preferred storage backend.
+
+### Cons
+
+- Kine can only be utilized with TLS due to the requirements of the API server.
+
+## Other changes needed in k3s to better support kine in rke2
+
+When testing rke2 with kine, there was some changes to avoid panics (specially when we are talking about `etcd`) and to make it work with tls. The changes are that when the user
+uses `--database-endpoint` and other flags related to `etcd only` nodes, we have to ignore this flags or simply end the process with a error message.
+
+### Pros of Ignoring the flags
+
+- It is possible to avoid panics and rke2 will run as expected.
+
+### Cons of Ignoring the flags
+
+- It will be not very clear to the user that the flags are being ignored.
+
+### Pros of Ending the process with a error message
+
+- Rke2 will run as expected with transparency to the user.
+
+### Cons of Ending the process with a error message
+
+- The user will have to change the flags to make rke2 run.