ENH: Add async tasks

[Miksus]

This PR adds async support (#28) for the main scheduler but also adds support for parallelizing tasks as async.

For example:

@app.task(execution="async")
async def 

Some notes:

• If async def ... is used with execution as main, the scheduler won't continue until the task is completed
• If the task itself is not using async def ... but just def ..., the task cannot be parallelized
• The task should release the executor every once in a while so that the scheduler can also do work

[codecov-commenter]

Codecov Report

Merging #60 (617de60) into master (49bbf4e) will increase coverage by 0.15%.
The diff coverage is 95.50%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   90.41%   90.56%   +0.15%     
==========================================
  Files          86       86              
  Lines        3892     3934      +42     
==========================================
+ Hits         3519     3563      +44     
+ Misses        373      371       -2     

Impacted Files	Coverage Δ
rocketry/core/schedule.py	89.96% <91.48%> (+0.14%)	⬆️
rocketry/application.py	82.08% <100.00%> (+1.13%)	⬆️
rocketry/core/task.py	92.10% <100.00%> (+0.48%)	⬆️
rocketry/session.py	91.20% <100.00%> (+0.91%)	⬆️
rocketry/tasks/func.py	93.83% <100.00%> (+0.12%)	⬆️

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[gaby]

Looking forward to this feature!

[Miksus]

This PR also fixes numeric timeout,