Add package setting to disable network isolation

[tiran]

In Fromager 0.31, network isolation is a global, process wide setting. There are cases where we want to disable network isolation for a specific package, because we know that a specific package does not build with network isolation. With a new package setting, we could disable network isolation for a package. It would also help us to discover if other packages later in the pipeline fail with network isolation

class BuildOptions(pydantic.BaseModel):
    network_isolation: bool | None = None

• None: use global settings
• False: force network isolation off
• True: force network isolation on IFF the system supports it