Add demo graph algorithm as a table function.

[andyfengHKU]

This PR a proof concept to demonstrate how to

• Add graph class as an abstract layer and pass it as the input of an algorithm function.
• Register algorithm function as a table function and query it with CALL

[codecov]

Codecov Report

Attention: Patch coverage is 19.54023% with 70 lines in your changes are missing coverage. Please review.

Project coverage is 92.13%. Comparing base (0b70b02) to head (cec08ae).
Report is 10 commits behind head on master.

❗ Current head cec08ae differs from pull request most recent head dc18b21. Consider uploading reports for the commit dc18b21 to get more accurate results

Files	Patch %	Lines
src/graph/function.cpp	10.63%	42 Missing ⚠️
src/graph/on_disk_graph.cpp	0.00%	27 Missing ⚠️
src/include/graph/graph.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3221      +/-   ##
==========================================
- Coverage   92.42%   92.13%   -0.29%     
==========================================
  Files        1164     1164              
  Lines       44103    44216     +113     
==========================================
- Hits        40762    40739      -23     
- Misses       3341     3477     +136     

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[semihsalihoglu-uw]

This is the class that I don't understand. What's "function/function.h vs graph/function.h"? I was expecting to see an algorithm.h class that forms an interface for algorithms, with a generic compute function? I understand Algorithms will be registered as TableFunctions but people implementing new algorithms in the system (us or eventual other developers who write extensions) should not know about the function infrastructure. So it looks like we need a higher-level abstraction over functions to develop algorithms. For example, it's confusing to me that the Graph field is part of DemoAlg's shared state (I am referring to the field in the struct DemoAlgoSharedState : public CallFuncSharedState in function.cpp.

I also need to understand how to pass arguments to functions, e.g., if we needed to pass a start source node to a ShortestPaths algorithm, how do we pass it and how much of that could be simplified. For example, can we abstract away from developers the details of binding expressions or writing any or minimal binding code? Similarly for output expressions of algorithms, I was imagining there is a Algorithm<AlgorithmResult> interface and AlgorithmResult has a factorized table as output, from which the output expressions can be read. Or something along those lines.