feat: pullable

[lwshang]

This PR simply adds the required pullable metadata.

To verify the overall workflow, we will need to make a new release so that the wasm module in GitHub release artifacts will have these metadata. And that release should not be "pre-release" in order to be accessible using "latest" label.

And there is an issue on the dfx side which requires wasm_hash_url to contains the hash only. Instead of modifying the CI of this project, I change the dfx deps pull to accept shasum output. PR is pending.

I removed the "pre-release" label of the 0.2.8 release. So that I could verify the wasm_url and wasm_hash_url can be reached.

And I also verified the content of init_guide field is correct to deploy the canister locally.

[THLO]

The linked PR is no longer pending.
What is still required to drop the Draft label?

[lwshang]

This PR is ready.

Notice that the sdk PR was merged but there is no dfx release including it yet. We will likely have one next week.