Add loading separate metadata files to cube attributes

[mo-tgeddes]

✨ Feature Request

Add the ability to load .attribution , .license and other metadata files into a cube's attributes alongside the cube.

Requirements

• will load metadata files alongside the cube
• will add the contents of the metadata files to the cube's attributes

Context

• This will preserve attributes in the processing chain.
• This will ensure that fieldsfile metadata is preserved if the data is then saved as a netcdf file.

[mo-tgeddes]

Other scope things:

• Should give a warning or error if metadata attributes are already there
• Should be for all filetypes
• Should only work for restricted list of types (but allow for spelling mistakes)
• Make sure pp files won't inadvertently be loaded as an attribute

Out of Scope/not needed:
-Landseamask files don't need this functionality