[host/{mem/region,sandbox{config,builder,uninit}] introduces SandboxBuilder

This commit partially introduces the SandboxBuilder mentioned on hyperlight-dev#249, changing
the way we create uninitialized sandboxes.

- adds SandboxBuilder API, which allows for a more flexible memory setup.
- changes to uninit w/ how it is created.
- deletes memory_region.rs file w/ most of it being superseded by the SandboxBuilder structures.
- simplifies config.rs removing all items related to previous static memory layout.

Signed-off-by: danbugs <[email protected]>

Author: danbugs