Make miden_lib::notes::build_swap_tag() public

[igamigo]

What should be done?

Currently, this function is private which is unfortunate for projects like the client which could make use of it. As of now, the function is duplicated in the codebase but we could remove it if we made it usable from outside of miden-lib

How should it be done?

Making the function pub.

When is this task done?

When the function can be used from outside of miden-lib

Additional context

0xPolygonMiden/miden-client#437 (comment)

[Fumuran]

Closed by #817.