Add hooks for enter and exit of top-level call host fn

[leighmcculloch]

What

Add hooks for enter and exit of top-level call host fn

Why

In rs-soroban-sdk#1158 we're adding auth to test snapshots so that after a test is run there's a snapshot of all the auths that have happened during the test on the environment. However, the host only stores auths for the last top-level call. The SDK will use the exit hook to take a snapshot of the auths and store them in the Env for later use when the test snapshot is generated.

Also, the SDK has some functions that only work inside a contract. It'd be helpful in the SDK if we could detect when we're not in a contract when those functions are called so we can display error messages that are more meaningful to SDK users rather than simply surface Host error messages. Enter and exit hooks of top-level calls would allow the SDK to track if we're inside a contract or not.

Blocking:

• Integrate top-level call enter/exit hooks into generation of test snapshots rs-soroban-sdk#1164
• Use Host has_frame logic in SDK to panic fns called when in or not in a contract as required rs-soroban-sdk#1165