feat: Implement before_connect callback to modify connect options.

[fiadliel]

Allows the user to see and maybe modify the connect options before each attempt to connect to a database. May be used in a number of ways, e.g.:

• adding jitter to connection lifetime
• validating/setting a per-connection password
• using a custom server discovery process

One approach to the issue in #3051
Also may help to fix #445

This is based on the signature suggestion from #3051. I'm not certain about the Error type used here; how should a user of the API represent an external error, sqlx::Error::Io?

Also, not sure how you would want this tested? The existing callback tests use a database connection which does not yet exist in this case.

[abonander]

@fiadliel if you rebase it should fix the build failure.

[fiadliel]

@abonander Rebased, thanks!

[abonander]

@fiadliel looks like the examples are failing to compile.

[fiadliel]

Fixed.

By the way, to show a concrete use case, I use this code to get a GCP access token (which has an expiry), this is then used as a password when connecting.

    PoolOptions::new()
        .before_connect(move |opts: &PgConnectOptions, _num_attempts| {
            let auth_manager = auth_manager.clone();
            Box::pin(async move {
                if let Ok(token) = auth_manager
                    .get_token(&["https://www.googleapis.com/auth/cloud-platform"])
                    .await
                {
                    Ok(Cow::Owned(opts.clone().password(token.as_str())))
                } else {
                    Err(sqlx::Error::Io(std::io::Error::other(
                        "failed to get token",
                    )))
                }
            })
        })

Updating the password in a background task is suboptimal, because (for various reasons) a background task is not guaranteed to run in time (e.g. there are systems that only run your code during an active request, and suspend them at other times).

[tmh-mfr]

Hey there! We are using SeaORM in a new project and would would love to have this feature. Same usecase as @fiadliel mentioned, just on the AWS side. We get an access token via IAM which is only valid for 15 min.

Is there anyway I can help with getting this merged?