fdc3.entity.fileAttachment is only defined inline in the fdc3.message schema and example has wrong type

[kriswest]

While working on generating context documentation from schemas I noticed that the fdc3.message type defines an fdc3.entity.fileAttachment type inline:

FDC3/schemas/context/message.schema.json

Line 46 in fdf060f

"const": "fdc3.entity.fileAttachment"

and the type name does not agree with that used in the example (fdc3.fileAttachment):

FDC3/schemas/context/message.schema.json

Line 89 in fdf060f

"type": "fdc3.fileAttachment",

FDC3 context types should each have their own definition and schema to make them usable and to properly reserve the type name. Hence, this definition should be separated out into its own schema and be referenced into fdc3.message.

@Yannick-Malins & @mistryvinay