Modify network-slice-assignment API Path and Feature Files

[XunliYang]

What type of PR is this?

Add one of the following kinds:

• enhancement/feature

What this PR does / why we need it:

• Reformat network-slice-assignment API Path
• Add the definition of notificationsBearerAuth in both API definitions
• Modify schema CreateSlice to SliceAttributes
• Modify description unbind --> release - consistent terminology helps understanding
• Modify schema SliceDevice --> SliceDevices (as the type is a list)
• Modify OperationId in API definition YAMLs (AssignDevice, releaseDevice, getDevices, retrieveSlicesByDevice)
• Modify feature filenames to consistent with operationId

Which issue(s) this PR fixes:

Fixes #94 #95

Special notes for reviewers:

Changelog input

 release-note

Additional documentation

This section can be blank.

docs