[lib] Re-introduce MemberInfo ([Legacy/MinimallyEncoded]MemberInfo)

Summary:
Replace usages of LegacyMemberInfo | MinimallyEncodedMemberInfo w/ re-introduced MemberInfo. Similar to D100068 but for MemberInfo.

Depends on D10071

Test Plan: flow/CI/etc

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10072