Allow variable values to be passed in on command line

[lisad]

Issue #152 suggested this feature idea

When working with data from different sources, different days, different URLs etc, the pipeline should be able to be launched with variable names

E.g: data team periodically downloads "employees.csv" from multiple companies and it's also important what day it's downloaded

> python3 -m phaser run employee_pipeline output "sources/employees.csv" --var company_name="Example" --var date="2024-06-06"

I guess this would be done like this:

• in the CLI, add an argument name="--var", with the 'append' action (using python's argparse library)
• split each value of 'var' on the equal sign
• after the pipeline is created, get its context and use add_variable for each value

[jeffkole]

Update phaser-example to use this new feature