Trivial RFE: add dspy.__version__ variable

[rpgoldman]

Not a huge problem, but it would be nice if we could introspect on the dspy version.

[okhat]

@hmoazam I wonder how easy for this to work with the way we publish versions?

[hmoazam]

Yes we can do this, I'll take a look.

[hmoazam]

Solved: #1963