Add xdrip_cgm customization and submodule-fetch support

[bjorkert]

Purpose

Add the Add xDrip4iOS as a CGM customization to CustomizationSelect, and add support for customizations that introduce a new submodule.

Pairs with the patch in loopandlearn/customization#65 (branch xdrip_cgm).

Method

• New menu entry add_customization "Add xDrip4iOS as a CGM (clones xdrip-client-swift)" "xdrip_cgm" ...
• add_customization gains an optional sixth argument: the path of a submodule the customization adds. When present, after the patch applies the script clones that repository using the URL the patch added to .gitmodules (so the referenced Xcode project exists), and removes the directory when the customization is reverted. This mirrors how the GitHub-action build clones xdrip-client-swift.
• Existing source-only customizations are unaffected (empty sixth argument).
• Regenerated CustomizationSelect.sh from src/.

Dependency

The patch itself lives in loopandlearn/customization#65. The test commands below fetch this branch's CustomizationSelect.sh and point PATCH_BRANCH at the matching customization branch.

Test

reset command

git submodule foreach 'git reset --hard; git clean -fd;'

apply

export PATCH_BRANCH="xdrip_cgm"
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/loopandlearn/lnl-scripts/xdrip_cgm/CustomizationSelect.sh)" \
    xdrip_cgm
unset PATCH_BRANCH

Confirm the patch applies, xdrip-client-swift/ is cloned, xdrip-client-swift/xDripClient.xcodeproj exists, and that reverting the customization both reverses the patch and removes the cloned directory.