Add additional priority queue related APIs

[bretambrose]

• aws_priority_queue_node_init - so that we're not adding magic SIZE_MAXs everywhere it's used
• aws_priority_queue_node_is_in_queue - so that we're not adding magic SIZE_MAXs everywhere it's used
• aws_priority_queue_clear - because sometimes you want to clear the queue without deallocating and re-initing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0ce8c56) 83.20% compared to head (f018a06) 83.20%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1067   +/-   ##
=======================================
  Coverage   83.20%   83.20%           
=======================================
  Files          55       55           
  Lines        5715     5729   +14     
=======================================
+ Hits         4755     4767   +12     
- Misses        960      962    +2     

Files	Coverage Δ
source/priority_queue.c	91.08% <100.00%> (+0.71%)	⬆️
source/task_scheduler.c	77.96% <100.00%> (ø)

... and 2 files with indirect coverage changes

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