Make TaskContext wrap SessionState

### Is your feature request related to a problem or challenge?

I have a user defined table function `FlattenTableFunc` whice needs reading data from other table provider. To serialize execution plan `FlattenExec`, I can't put `SessionState` in it directly. So I have to reconstruct `SessionState` in `FlattenExec::execute` method.

### Describe the solution you'd like

I see `TaskContext` is just part of `SessionState`. Could we just make `TaskContext` wrap `SessionState`?

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reading the TaskContext and SessionState definitions, then inspect the FlattenExec::execute entry point described in the issue. Determine how execution-plan serialization and access to table providers should work when TaskContext wraps SessionState; done means the execution path no longer reconstructs SessionState in FlattenExec::execute.

Written by the indexing model from the issue text.