(redo) feat: added new setting CUSTOM_RESOURCE_TEMPLATES_DIRECTORY

[keithgg]

This PR is a redo of #30759 which itself is a backport of #30108 to maple.master. Re-using the description below.

Description

At Opencraft, we have clients that want to be able to add custom HTML templates that show up while creating course components in Studio.

Currently, there's no way to achieve this besides forking edx-platform and adding the yaml files to common/lib/xmodule/xmodule/templates/

This PR allows operators to set a custom directory where templates can be additionally loaded from.

• Checks if settings.CUSTOM_RESOURCE_TEMPLATES_DIRECTORY exists.
• If so, load the yaml files with the same logic as in the xmodule/templates directory.

Supporting information

• Configuration PR: feat: added new setting CUSTOM_RESOURCE_TEMPLATES_DIRECTORY openedx-unsupported/configuration#6706
• Custom Client Branch: [SE-5135] Update HTML templates to the v2 counterparts. open-craft/openedx-platform#453
• Task: https://tasks.opencraft.com/browse/BB-5885
• Sandbox: https://studio.pr30108.sandbox.opencraft.hosting [Sandbox]

Testing instructions

• Update the setting settings.CUSTOM_RESOURCE_TEMPLATES_DIRECTORY to a valid path.
• In that directory create an html directory.
• Copy a template from common/lib/xmodule/xmodule/templates/html to the above directory with a different filename.
• In the Studio, edit a Unit. Under the Text component you should see your new template list (on the Sandbox I've added many).

Deadline

"None"

Other

Settings

EDXAPP_CUSTOM_RESOURCE_TEMPLATES_DIRECTORY: /edx/var/edx-themes/edx-themes/resource-templates
edx_platform_commit: keith/bb_5885_maple_backport
edx_platform_repository_url: https://github.com/open-craft/edx-platform.git
configuration_source_repo_url: https://github.com/open-craft/configuration.git
configuration_version: master

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