SlurmManager: Add the custom_worker_flag option, but don't add it to the public API

[DilumAluthge]

Description

I have a use case where, instead of starting each remote worker with julia --worker, I need to start the remote workers with julia -e 'my_custom_code()'.

This PR modifies SlurmManager to have a custom_worker_flag option. When custom_worker_flag is specified, it will replace --worker. When custom_worker_flag is not specified (which is the default), it falls back to the current behavior.

Example usage

using Distributed: addprocs
using SlurmClusterManager: SlurmManager

julia_code = """
    my_custom_code()
"""

addprocs(SlurmManager(); custom_worker_flag = `-e "$(julia_code)"`) 

This will start each remote worker with julia -e 'my_custom_code()'.

Public API vs private API

This PR does not add the new custom_worker_flag option to the public API. This allows us to change (or remove) it in the future.

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 87.03%. Comparing base (e49016d) to head (674f016).

Files with missing lines	Patch %	Lines
src/slurmmanager.jl	70.58%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   90.42%   87.03%   -3.39%     
==========================================
  Files           2        2              
  Lines          94      108      +14     
==========================================
+ Hits           85       94       +9     
- Misses          9       14       +5     

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[DilumAluthge]

This is not ready to merge yet, but I figure it can't hurt to get an early round of reviews.