[mlir][transform] Support for multiple top-level transform ops (#69615)

This adds a flag to the `TransformDialectInterpreter` that relaxes the
requirement for only a single top-level transform op.
This is useful for supporting transforms that take transform IR as
payload.

This also aligns the function `findTopLevelTransform`
[here](https://github.com/llvm/llvm-project/commit/7b0f4c9db55c355bffddf94d7710f40ee2c1e9db#diff-551f92bb609487ccf981daf9571f0f1b1703ab2330560a388a5f0d133e520be4L59)
with its documentation:
In the presence of multiple top-level transform ops it now correctly
returns the first of them after reporting the error instead of returning
a `nullptr`.