Add support for 'alwaysOutOfDate' PBX Shell Script property

[marco-saia-datadog]

This PR adds support for the alwaysOutOfDate property in PBX Shell Scripts.

Setting alwaysOutOfDate to true disables dependency analysis for the given build phase, clearing warnings such as:

Script has ambiguous dependencies causing it to run on every build

Since by default the property is not added to the PBX project file by Xcode, I have decided to write it down in the PBX file only if explicitly specified in the pbxShellScriptBuildPhaseObj options, hence the null check.