[loader] Expose TypeTag to Type conversion

[awelc]

Motivation

Currently, we have the ability to convert Type to TypeTag in native context but not vice versa which may be useful if in the implementation of a native method you need to created types (struct types specifically) out of thin air, such as in https://github.com/MystenLabs/sui/pull/8120.

Have you read the Contributing Guidelines on pull requests?

Yes

[awelc]

Converted to draft as apparently exposing this is a big no-no...