DAC: add Dac_plugin.raw_hash type in the APIs

Context

Dac_plugin.hash is a plugin-related type. This means the plugin is needed to correctly decode/encode it.

A trouble would happen if, for example, Coordinator and Member are not using the same plugin.

Solution

Introduce a Dac_plugin.raw_hash type, which is basically an alias for byte and using a "generic" encoding (non plugin related). Then, use this new type in every endpoints.

This will ensure every actors are dealing with the exact same encoding at every moments.

Inside this MR

• Define new type Dac_plugin.raw_hash
• Define functions to convert and deal with this type
• Define transformation function to convert from/to Dac_plugin.hash
• Make every endpoint using Dac_plugin.raw_hash
• Get rid of useless Dac_plugin.t parameter in several functions (mostly endpoint)

Closes #4895 (closed)