Add shell to ExecuteAction

[balazs-vimn]

1429 Add shell parameter to Execute Action

[yonaskolb]

Thanks @balazs-vimn

Could you please add:

• encoding: https://github.com/yonaskolb/XcodeGen/blob/2c1500761dc91ca742ce73602615299e499d23ce/Sources/ProjectSpec/Scheme.swift#L407
• decoding: https://github.com/yonaskolb/XcodeGen/blob/2c1500761dc91ca742ce73602615299e499d23ce/Sources/ProjectSpec/Scheme.swift#L399