Implement transaction options API

[andrewwhitehead]

This adds begin_with methods for creating transactions with additional options. The exact options object varies depending upon the backend. For MS-SQL and AnyDatabase the options are currently just () although this could be extended, with each backend transaction option struct potentially supporting From<AnyTransactionOptions>.

MySQL and Postgres both support the same ANSI isolation levels, although they are currently separate enums which could potentially be combined.

As much as possible, the tests try to verify that the correct transaction options have been applied.

Related to #481, #853, #851

[abonander]

This is good but I have a number of nits.

I still think something like the what I proposed here would be good in addition to this: #481 (comment) to allow for any options we don't currently support.

[abonander]

I'm sorry, I should have looked at this PR before releasing 0.6.0 because this is a breaking change.

While it's unlikely that any implementation of this trait exists outside SQLx, since we haven't sealed any of them it's not impossible.

[abonander]

I think I would prefer to see at least an empty MssqlTransactionOptions type so that we can safely add things to it later without it being a breaking change.

[abonander]

Actually, I think this could be a struct containing a TransactionOptions for each database, and then the implementation picks the corresponding one:

#[derive(Default)]
pub struct AnyTransactionOptions {
    #[cfg(feature = "postgres")]
    postgres: PgTransactionOptions

    #[cfg(feature = "mysql")]
    mysql: MySqlTransactionOptions,

    // etc.
}

[abonander]

This could then have setters to replace each type wholesale:

impl AnyTransactionOptions {
    #[cfg(feature = "postgres")]
    pub fn postgres(self, postgres: PgTransactionOptions) -> Self {
        Self { postgres, ..self }
    }

    // etc.
}

Or even have setters for "best effort" options:

impl AnyTransactionOptions {
    pub fn isolation_level_if_supported(self, ilvl: IsolationLevel) -> Self {
        #[cfg(feature = "postgres")]
        self.postgres = self.postgres.isolation_level(ilvl);

        #[cfg(feature = "mysql")]
        self.mysql = self.mysql.isolation_level(ilvl);

        self
    }
}

(Since PgIsolationLevel and MySqlIsolationLevel are identical, they don't need to be separate enums.)

[xfbs]

This looks really awesome! As #481 indicates, there is currently no way to do a BEGIN IMMEDIATE TRANSACTION for SQLite databases in SQLx (if I understand things correctly), and we need that functionality. I hope this can be merged soon!

[andrewwhitehead]

Thanks for the review @abonander, it sounds like this might need to wait for another minor release? I see there's another PR applying the feedback.

[abonander]

Closing in favor of #1924