Add Degree graph algorithm

[Adityaaprakash]

What

Adds a built-in degree() graph algorithm.

Output format:

• id
• in_degree
• out_degree

How

• Implements Degree as a non-iterative graph algorithm.
• Uses loadEdges(EdgeDirection.IN) and loadEdges(EdgeDirection.OUT).
• Does not perform message passing.
• Registers the algorithm in the built-in function table.
• Adds runtime test coverage.

Verification

• Added runtime SQL test.
• Added expected output verification.
• Followed existing graph algorithm patterns.

Fixes #795