Add SandboxBuilder

`SandboxBuilder` is the entry point for creating a sandbox. It gathers machine
configuration, host functions, init data and memory mappings, then builds a
`MultiUseSandbox` from a guest binary on disk, a guest binary in memory, or a
snapshot.

It merges the roles of `SandboxConfiguration`, `GuestEnvironment` and
`UninitializedSandbox` into a single type, so all three can become
implementation details.

Every setting has an in place accessor taking `&mut self` and a `with_*`
counterpart consuming `self` for chaining. Machine configuration values are
readable through `get_*`.

`map_memory_region` is `unsafe`, matching `MultiUseSandbox::map_region`. The
mapped region must stay valid for the lifetime of the built sandbox.

`build_from_snapshot` errors when `init_data` or `max_guest_log_level` are set,
as a snapshot already carries both.

Author: jprendes

src/hyperlight_host/src/lib.rs

@@ -91,6 +91,8 @@ pub use hypervisor::virtual_machine::is_hypervisor_present;
 pub use sandbox::MultiUseSandbox;
 /// The re-export for the `UninitializedSandbox` type
 pub use sandbox::UninitializedSandbox;
+/// The re-export for the `SandboxBuilder` type
+pub use sandbox::builder::SandboxBuilder;
 /// A collection of host functions that can be supplied to a sandbox
 /// constructor (e.g. [`MultiUseSandbox::from_snapshot`]).
 pub use sandbox::host_funcs::HostFunctions;