Update cugraph python code to use constants for various dataframe column names

[rlratzel]

The current cugraph python code still has many places where hardcoded column names are used, sometimes inconsistently (eg. src, dst, weight, weights, wgt).

Other column names from PropertyGraph include those for edge ID, edge type, and vertex type, and may also need to be considered.

The proposal here is to create a single python module that defines the column names as constants which can be imported and used throughout the python code.

This PR addresses a similar problem, and should be considered in the final solution.

[github-actions]

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