Handling of optional parameters

[kibertoad]

gRPC provides two mechanisms for handling optional parameters: Structs and FieldMasks. I couldn't find anywhere in documentation whether either of them are used for attributes that are specified as non-mandatory in Swagger. There doesn't seem to be any examples either.

Two main use-cases for this are PATCH and get?attrName=attrValue (find) operations, where you definitely want to distinguish between defined and undefined values, since searching by "name='John' && age = 0" is not the same as "name='John' && age = undefined"

[kibertoad]

This #379 is related, but PATCH is only part of the story here :)

[kibertoad]

Created #698 for GET and now I guess this one is redundant.