feat: Add secret variable overrides to SDK and CLI

[andyland]

Adds sdk support for overriding secret environment variable names

Example usage:

  lightning deployment create service-a \
    --teamspace org-9vmve/general \
    --image nginx \
    --machine CPU \
    --port 80 \
    --secret TEST=TEST_A

  lightning deployment update service-a \
    --teamspace org-9vmve/general \
    --secret TEST=TEST_B

  from lightning_sdk import Deployment, Machine
  from lightning_sdk.deployment import Secret
  
  dep_a = Deployment(name="service-a", teamspace="org-9vmve/general")
  dep_a.start(image="nginx", machine=Machine.CPU, ports=80,
              env=[Secret("TEST_A", env_name="TEST")])
  
  dep_b = Deployment(name="service-b", teamspace="org-9vmve/general")  dep_b.start(image="nginx", machine=Machine.CPU, ports=80,
              env=[Secret("TEST_B", env_name="TEST")])

Loom: https://www.loom.com/share/a27842241e354477b65dd5961462d643