Move remaining models out of portability-types-transfer and into portability-types-common

[seehamrun]

Theres documentation and content about the Blobby model in portability-types-transfer whereas the rest of the models live in portability-types-common.

https://github.com/google/data-transfer-project/tree/abb554b604329dcacaa62a8b8c5615f6aa47e0bd/portability-types-transfer/src/main/java/org/datatransferproject/types/transfer/models

https://github.com/google/data-transfer-project/tree/abb554b604329dcacaa62a8b8c5615f6aa47e0bd/portability-types-common/src/main/java/org/datatransferproject/types/common/models

[jzacsh]

reopening (looks like the documentation was missed - see #1056 - but there might be more string matches like this too, so I'm just reopening to do another check)