[lib] Expand MemberInfo to LegacyMemberInfo | MinimallyEncodedMemberInfo

Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedinfo-to-rawinfo

We won't really be able to remove usages of LegacyMemberInfo, but this will at least let us rename MinimallyEncodedMemberInfo to just MemberInfo.

Depends on D10734

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10789