Add support for coloring the logo in the environment indicator navigation module.

Problem/Motivation

While working on ✨ Support for core navigation experimental module Needs work @jrb added a compelling suggestion ✨ Support for core navigation experimental module Needs work to allow coloring the logo.

Adding this to the new modules/environment_indicator_navigation/css/navigation.css file would make the Drupal logo the specified color, too (rather than the default blue):

/* Navigation logo colors. */
.admin-toolbar__logo svg rect[fill] {
fill: var(--env-indicator-bg-color);
}
.admin-toolbar__logo svg path[fill] {
fill: var(--env-indicator-fg-color);
}

Steps to reproduce

Proposed resolution

@svendecabooter suggested that we make coloring the logo optional, which is the impetus for this new issue.
https://www.drupal.org/project/environment_indicator/issues/3457688#comm... ✨ Support for core navigation experimental module Needs work

Remaining tasks

Create a new library for coloring navigation logos.
Create a new CSS file to be included in the library.
Create a new config schema file for environment_indicator_navigation that defines the new option.
Add environment indicator navigation option form to environment indicator settings form.
Add tests.

User interface changes

Site admins can optionally color the logo per environment.

API changes

Data model changes

New option for coloring the navigation logo.