Add DaggerGraphs subpackage

[jpsamaroo]

This was started during JuliaCon with @pszufe but I never finished it - it's a naive Graphs.jl directed graph implementation that uses Dagger to partition the graph.

@gdalle

Todo:

• Optimize add_vertices! implementation
• Add undirected graph support
• Add freeze for creating immutable DGraph
• Optimize immutable DGraph operations
• Default to directed graph
• Add vertex/edge metadata support
• Add lazy weights implementation (calls weights on metadata)
• Rename "external" to "background" for cross-partition AdjList
• Switch AdjList to dense representation (like SimpleGraph)
• Optimize purely local operations
• Validate Graphs.jl interface compatibility
• Return and cache neighborhood on remote vertex lookup (inneighbors/outneighbors)
• Implement user-configurable caching strategies
• Prefetch neighborhoods
• Implement batched and/or streaming edge creation
• Add loading/saving from $k^2$ files
• Validate benchmark performance on purely-local operations versus SimpleGraph
• Add docs
• Add CI
• (Optional) Add support for ingesting vertices/edges from a DataFrame or Tables source