[Suggestion] Re-export `StructTag` from `move-core-types` in `sui-types`

[macaujack]

When we create a PTB (using ProgrammableTransactionBuilder), sometimes we need to fill the type parameters, which is of type Vec<TypeTag>.

TypeTag comes from move-core-types crate, and is re-exported in sui-types, which could be imported by use sui_sdk::types. This is convenient. However, TypeTag may wrap a StructTag, and StructTag is not re-exported. So I have to add move-core-types in my Cargo.toml.

It's kinda weird TypeTag is re-exported while StructTag isn't...

[github-actions]

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[stefan-mysten]

@macaujack this sounds reasonable, given that we already export some types from external-crates. I will put up a PR shortly.