Add qk_dag_successors and qk_dag_predecessors to the C API

[eliarbel]

This commit adds qk_dag_successors and qk_dag_predecessors to retrieve the neighbors of a given DAG node.
The information is returned through a new output struct called QkDagNeighbors. The struct data is cleared using the new qk_dag_neighbors_clear function.

Closes #15189

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Eric-Arellano
• @abbycross
• @beckykd
• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 19407009803

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 50 of 51 (98.04%) changed or added relevant lines in 2 files are covered.
• 15 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.005%) to 88.204%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/cext/src/dag.rs	40	41	97.56%

Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/parameter/parameter_expression.rs	1	81.99%
crates/qasm2/src/lex.rs	6	92.03%
crates/circuit/src/parameter/symbol_expr.rs	8	72.9%

Totals
Change from base Build 19386973715:	0.005%
Covered Lines:	94287
Relevant Lines:	106897

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💛 - Coveralls