[App] Introduce auto scaler

[akihironitta]

What does this PR do?

Introduces AutoScaler.

---

Example 1 (autoscale out of the box)

import lightning as L

# without auto scaling
app = L.LightningApp(ServeSomething())

# with auto scaling
app = L.LightningApp(
    L.app.components.AutoScaler(
        ServeSomething,
        min_replicas=1,
        max_replicas=4,
        ...,
    )
)

Example 2 (customize the scaling logic)

import lightning as L

class MyAutoScaler(L.app.components.AutoScaler):
    def scale(self, replicas, metrics) -> int:
        # upscale
        if metrics["num_requests"] > 10:
            return replicas + 1
        # downscale
        elif ...:
            return replicas - 1

app = L.LightningApp(
    MyAutoScaler(
        ServeSomething,
        min_replicas=1,
        max_replicas=4,
        ...,
    )
)

Does your PR introduce any breaking changes? If yes, please list them.

None

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Did you have fun?

Make sure you had fun coding 🙃

(Not sure if this app feature PR should be part of the 1.9 milestone or 1.8.x.)

cc @Borda

[tchaton]

LGTM !

[akihironitta]

Blocked by the failure of cloud-e2e tests running on Azure.