feat: add s3 retention config for etcd snapshots

[github-actions]

This pull request cherry-picks the changes from #2337 into release/v15

Addresses #2339 for #2338

WARNING!: to avoid having to resolve merge conflicts this PR is generated with 'git cherry-pick -X theirs'.

Please make sure to carefully inspect this PR so that you don't accidentally revert anything!

Copied from main PR:

Rancher now supports a separate retention count for S3-stored etcd snapshots, independent of the local snapshot_retention value.

• Addresses [PROBLEM] ETCD s3_config doesn't support snapshots retention setting #2126

Description

Testing

My current cluster is configured with the following etcd configuration in rancher2_cluster_v2.rke_config.etcd

    etcd {
      snapshot_retention = 2
      ...
      s3_config {
        ...
      }
    }

Updating the provider doesn't make any changes to the existing cluster configuration.

Next updating the provider and setting retention = 3 results in the following changes

  # module.downstream_cluster[0].module.cluster.rancher2_cluster_v2.main will be updated in-place
  ~ resource "rancher2_cluster_v2" "main" {
        id                           = "..."
        name                         = "..."
        # (10 unchanged attributes hidden)

      ~ rke_config {
            # (3 unchanged attributes hidden)

          ~ etcd {
                # (3 unchanged attributes hidden)

              ~ s3_config {
                  ~ retention             = 0 -> 3
                    # (7 unchanged attributes hidden)
                }
            }

            # (10 unchanged blocks hidden)
        }

        # (1 unchanged block hidden)
    }

Not a breaking change.