feat: add set custom primary colors method in initialize flow

[andrey-canon]

Description:

This allows to set css custom properties, in this case just primary colors, in order to take advantage of this changes eduNEXT/paragon#1

How to test

1. install this paragon version https://github.com/openedx/paragon/tree/alpha
2. set mfe api config feat: MFE configuration at runtime openedx/frontend-platform#335
3. Add to your lms config the following:

    "ENABLE_MFE_CONFIG_API": true,
    "MFE_CONFIG": {
        "CUSTOM_PRIMARY_COLORS": {
            "pgn-color-primary-base": "#AA0000"
        }
    },

Result will be something like this

[codecov-commenter]

Codecov Report

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Additional details and impacted files

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[felipemontoya]

As we talked via other channels, I think this is great and it would be even better if this allowed for the optional inclusion of a css variables file.

This way we can continue developing the ways in which to get you the css-variables without the need for a new deployment everytime.

[dcoa]

This should be an object instead of a string property

[dcoa]

This function doesn't have any process async then you should remove this world

[dcoa]

Await is not necessary here because the function doesn't return a promise.