sp-api: Support nested transactions (#14447)

* sp-api: Support nested transactions

Adds support for nested transactions in `sp-api` by using `execute_in_transaction`. This was working
until a recent refactor, but this was actually not intended. However, supporting nested transactions
is a worthwhile feature to have. So, this pr "brings it back" and adds a test to ensure it will not break.

* Make clippy happy

* Assert that the runtime api type is not unwind safe

* Count number of transactions