Swupd update should have a --dry run option to show what changes in an update, size

[karthikprabhuvinod]

Swupd update should have a dry run option.

Rationale: It is good to know what changes while moving from one version to another without actually changing it.

• size of the update
• new bundles that will be created, bundle that will be removed or changed

[otaviobp]

This issue is related to #1151 It's important to have the alternative defined before moving forward with the implementation.