wip: add an API of plugins to retrieve authentication information

[tomerd]

motivation: some plugins need authentication information, having an API to retrieve it from SwiftPM is helpful to abstract different authentication providers

changes:

• create a new API for plugins to request authentication information from SwiftPM
• implement the IPC encoding/decoding for the API

TODO:

• encrypt IPC so that auth info cannot be man-in-the-middle
• add plugin privilege to signal a plugin would access auth information
• add tests

rdar://116226961

[MaxDesiatov]

cc @euanh