chore(storage): rename conversions

[joseivanlopez]

• Rename conversions from ToSchema and FromSchema to ToJSON and FromJSON.
• The namespaces ProposalSettingsConversion and VolumeConversion are renamed as ProposalSettingsConversions and VolumeConversions.
• Conversion methods are moved from the namespace modules to the ProposalSettings and Volume classes.

Note: conversions under Agama::DBus::Storage namespace are not modified because they are going to be removed in future iterations.

[coveralls]

coverage: 71.4% (-0.01%) from 71.41%
when pulling a3bc41e on rename-schema-conversion
into accf894 on master.