feat: separation - endpoints for finalize & assign

finalize at:
    POST /v1.0/vaults/{vaultid}/files/{fileid}
assign at:
    POST /v1.0/vaults/{vaultid}/files/{fileid}/blocks
Support update to assign/finalize file endpoint
Support for x-file-id header
Merge remote-tracking branch 'upstream/master' into separate_endpoint_assingn_finalize

Conflicts:
	deuce/tests/test_files.py
Merge pull request #199 from TheSriram/separate_endpoint_assingn_finalize

feat: separation - endpoints for finalize & assign
Merge pull request #203 from jc7998/assign

API Test: Support update to assign/finalize file endpoint
Merge pull request #204 from jc7998/fileid

API Test: Support for x-file-id header