Defining a PyRosetta build signature specifying extras (#537)
This quick PR defines a unique PyRosetta build signature that differs
from the existing PyRosetta version string in that it contains build
extras as given by `rosetta.utility.Version.extras()`. The aim is to
capture build differences in the official PyRosetta `ml` images from
non-`ml` images. The `PyRosettaCluster(pyrosetta_build=...)` default
keyword argument parameter, and the PyRosetta `.init` file
`pyrosetta_build` key value, are updated herein accordingly.
Here is an example of the new `pyrosetta._build_signature()` function's
output:
```
PyRosetta4.Release.python311.mac.cxx11thread.serialization[extras:cxx11thread+serialization]2025.19+release.1354d05daa4c339d591afeecef3c94ca2d38680e
```