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

Summary:
Similar to D10068 and D10072, but for RelativeMemberInfo.

Note: There are a few places that we haven't updated to support MinimallyEncodedRelativeMemberInfo yet (specifically getRelativeMemberInfos). Those will be handled later in the stack (and will be easier to identify because we'll be able to search for LegacyRelativeMemberInfo and find all instances.

Depends on D10073

Test Plan: flow/CI/etc

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10074