refactor(kubernetes): streamline provider configuration and in-cluster detection (#378)

* refactor(kubernetes): streamline provider configuration and in-cluster detection

- Removed IsInCluster method from Manager and created function scoped to the runtime environment.
  As a method, the implementation was not correct.
  Removed GetAPIServerHost method from Manager which is no used.
- **Temporarily** added an `inCluster` field to the Manager struct but should be eventually removed since it doesn't really make sense to hava a Manager in-cluster or out-of-cluster in the multi-cluster scenario.
- Provider resolution (resolveStrategy) is now clearer, added complete coverage for all scenarios.
- Added additional coverage for provider and manager.

Signed-off-by: Marc Nuri <marc@marcnuri.com>

* refactor(kubernetes): update NewManager to accept kubeconfig context and simplify manager creation

- Removes Provider.newForContext(context string) method.

Signed-off-by: Marc Nuri <marc@marcnuri.com>

---------

Signed-off-by: Marc Nuri <marc@marcnuri.com>

internal/test/env.go

@@ -0,0 +1,15 @@
+package test
+
+import (
+	"os"
+	"strings"
+)
+
+func RestoreEnv(originalEnv []string) {
+	os.Clearenv()
+	for _, env := range originalEnv {
+		if key, value, found := strings.Cut(env, "="); found {
+			_ = os.Setenv(key, value)
+		}
+	}
+}