(assertions): argument to skip bundling of assets

### Description

When running `assertions.Template.from_stack(stack)`, one cannot skip the bundling of assets during synthesis/generation of the CloudFormation template.

### Use Case

When writing unit-tests that leverage the `assertions` library, assets do not always have to be created to check the synthesized template. Depending on how many assets are created (and their bundling strategy), the process can be lengthy. This feature-request is for an argument which can disable bundling of assets.

### Proposed Solution

`assertions.Template.from_stack(stack, bundle_assets=False)`

### Other information

I tried doing something like below, to no avail:

```python
@jsii.implements(IAspect)
class MockAssetStaging:
def __init__(self, fake_stage: AssetStaging):
self.fake_stage = fake_stage

def visit(self, node):
if isinstance(node, AssetStaging):
node = self.fake_stage

app = App()
core_stack = CoreStack(...)

fake_staged_asset = AssetStaging(core_stack, "fake", source_path=f"./hello_world.py")
Aspects.of(app).add(MockAssetStaging(fake_staged_asset))
```

### Acknowledge

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Start at assertions.Template.from_stack(stack), then trace how stack synthesis invokes asset bundling. The change is complete when the proposed bundle_assets=False argument lets assertion-based template generation skip asset bundling while preserving the existing default behavior; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.