FIX: Make types public

[Ali-Usama]

This PR makes sc-network-sync types public following this issue.

Fixes #3556

[cla-bot-2021]

User @Ali-Usama, please sign the CLA here.

[dmitry-markin]

Hmm, not sure if this should be merged into release branch and not in master. Normally we merge changes into master first and let them propagate into next release.

CC @altonen

[Ali-Usama]

Hmm, not sure if this should be merged into release brunch and not in master. Normally we merge changes into master first and let them propagate into next release.

CC @altonen

But I need this to be updated in release-polkadot-v1.6.0, and it's gonna take me a lot of refactoring just to go around this simple fix :(

[altonen]

I don't think it should be merged to release-polkadot-v1.6.0. I understand why want it in 1.6.0, and granted it's a trivial change, but I don't know if this is something we can bother the release people with.

@ggwpez do you have any thoughts?

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5424419

[Ali-Usama]

@altonen I'm gonna go ahead and close this PR and create a new one to be merged in the master branch