[services-lib] Add DDB Attribute conversion traits

Summary:
Introduced traits that allow for implementing convenient conversions from DDB AttributeValue to various types.
They will be implemented in subsequent diffs.

Depends on D8787

Test Plan:
Following bash one-liner:

cd services
for r in "comm-services-lib" "backup" "blob" "feature-flags"; do pushd $r; cargo check --all-features; popd; done

Reviewers: michal, jon, varun

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8789