Support for finally in PDL

[vazirim]

Is your feature request related to a problem? Please describe.
When a program fails in the middle, it's useful to be able to take an action in the end no matter how the execution went.

Describe the solution you'd like
A finally feature would execute at the end of a list of blocks, even if a failure occurs in the middle.

Describe alternatives you've considered
N/A

Additional context
This is useful for example to print the token usage in the end.

This issue was proposed by @eloycoto.