Fastnum decimals integration

[neogenie]

I've announced a new fast, exact precision decimal numbers crate fastnum.

The current integration with SQLx is implemented in the fastnum crate itself under the feature flag sqlx and
works fine only for PostgreSQL.I would like to move it inside SQLx. Can I expect on your help in PR review and
reserved feature flag in sqlx à la fastnum?

[abonander]

My disposition is the same as #3487 (comment)

See the linked comment for discussion on how jiff is integrating with the ecosystem as an example to follow.