Option to override SpaceDock version with AVC version

[HebaruSan]

Problem

A mod that exists only as a bundled folder in another mod on SpaceDock can't have its own version number in CKAN; it must use the bundling mod's version (see #2405). This misrepresents to the user what's being installed, and makes for an awkward transition if we have to switch the source mod, often requiring an epoch bump.

Cause

A decision was made to never trust version files if we have another source of version info:

CKAN/Netkan/Transformers/AvcTransformer.cs

Lines 154 to 160 in a495f43

	if (avc.version != null)
	{
	// In practice, the version specified in .version files tends to be unreliable, with authors
	// forgetting to update it when new versions are released. Therefore if we have a version
	// specified from another source such as SpaceDock, curse or a GitHub tag, don't overwrite it.
	json.SafeAdd("version", avc.version.ToString());
	}

Changes

Now there is a way to override the above behavior. If "x_netkan_trust_version_file" is set to true in a given netkan file, then the "version" property will be sourced preferentially from the version file, if any.

Fixes #2405; my test version generated DynamicBatteryStorage-2-1.3.0.0.ckan from the current netkan with that property added, which is the correct version from that bundled mod's version file.