Allow setting credentials as lambda

[laurynas]

We had a problem with multiple Puma workers and preload, because Temporal initializer which configures connection credentials ran before fork. Therefore after fork it we got an error:

grpc cannot be used before and after forking

Reconfiguring credentials after forking does not work because of the guard in grpc gem:
https://github.com/grpc/grpc/blob/e20e7a1198b2ff276a2f4f8514201b8ac1456830/src/ruby/ext/grpc/rb_grpc.c#L264
There is an old issue about it: grpc/grpc#8798

Therefore the most elegant solution was to configure credentials on demand, when connection is being built (after fork).
I have added a lambda support to solve this issue. It works for us.

@egletam

[lorines]

@laurynas @egletam any updates on when this will be merged?

[laurynas]

@lorines I don't know, it is up to the library maintainers.
cc @jeffschoner