Example of parsing SubMessage data field

[ethanfrey]

Builds on #502 / #480

Let's show a live example somewhere using these field to parse a SubMessage response. This can be only in test code, maybe in multitest. Best if used in production code. Maybe rust doc??

Point is something to link to to show how to use SubMessage to instantiate a contract and get it's address.

[maurolacy]

After the new cw-plus (v0.10.1) is released, this can be shown by refactoring tgrade-valset to use the new helpers in #480.

Then we can copy / reference that in the docs.

[ethanfrey]

I'd like to update something we have already open sourced, so I can link it this week

[maurolacy]

OK, I can take this after you cut the new release.

[maurolacy]

There's a case in multi-test where prost is currently being used. I'll change that to use the new helpers, and then we can use that as an example if you like / prefer.

[ethanfrey]

Sounds good.
Thank you

[maurolacy]

See #515. Example is pretty simple, not sure it will be useful for what you want.

[ethanfrey]

I think it is good enough. At least some working code I can link to.