SSHManager: 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 SSHManager 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

hosts = ["host1", "host2", "host3"]

julia_code = """
    my_custom_code()
"""

addprocs(hosts; 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 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.29%. Comparing base (ff8689a) to head (c9f786e).

Files with missing lines	Patch %	Lines
src/managers.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   79.42%   79.29%   -0.14%     
==========================================
  Files          10       10              
  Lines        1949     1951       +2     
==========================================
- Hits         1548     1547       -1     
- Misses        401      404       +3     

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[JamesWrigley]

Could you give some more details on what the usecase is? I'm not necessarily opposed to it, it just seems like a very easy footgun.