feat(cli): add AgentTemplate apply command (#2643)

## Summary

- add client-side apply commands for AgentTemplate manifests
- route lifecycle operations through the generated AgentTemplate gRPC
client
- add unit and E2E coverage for the full manifest lifecycle

## Tests

- make -C go lint
- KIND_CLUSTER_NAME=kagent-cli-refactor
KAGENT_E2E_GRPC_TARGET=127.0.0.1:18084 make -C go e2e

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Signed-off-by: Cody Hartsook <cody.hartsook@solo.io>