Support bigdecimal 0.4

[Sytten]

Motivation

Right now it's pinned to 0.3, but we can't force by default third party dependencies to use that version if they have looser requirements. So by default Diesel force example resolves to 0.4.1 which breaks the binder because traits are not matching. I think a >=0.3, <0.5 would be perfectly fine.

[joshburgess]

This is causing problems in my production codebase at work while trying to update sqlx, because the latest version of sqlx is using bigdecimal 0.4. (Mixed codebase with plain sqlx usage and sea-query usage)

If I try to use bigdecimal 0.3, I'm missing trait implementations required by sqlx. If I try to use 0.4, I'm missing trait implementations required by sea-query.

[tyt2y3]

Upgrading sqlx to 0.8 is in the works. #798