feat: Unify Debug implementations across PgRow, MySqlRow and SqliteRow

[davidcornu]

Context

In a personal code base I currently maintain a Row implementation called DebugRow as a catch-all type for queries so that I can use its Debug implementation for snapshot testing (using the great insta crate).

While upgrading the version of sqlx I noticed there was now a useful Debug implementation for PgRow (#2917) which had me hoping there was an equivalent for SqliteRow, but after digging through the code I found out

• PgRow has a custom Debug which prints out the contents of the row using debug_map()
• MySqlRow uses a derived Debug
• SqliteRow does not implement Debug

Rather than implement similar logic to PgRow for SqliteRow I figured it might be better to unify the implementations as the various traits (e.g. Row, Database, TypeChecking) provide everything needed to implement PgRow's version generically.

Also related: #150

Proposed changes

1. Introduce a new sqlx_core::row::debug_row function
2. Use (1) to implement Debug on SqliteRow
3. Use (1) to replace PgRow's Debug implementation
4. Use (1) to replace MySqlRow's derived Debug

Is this a breaking change?

Yes.

1. PgRow's Debug implementation changes a little bit because of the use of std::any::type_name
   (This could be avoided by passing a name to debug_row instead but is definitely less elegant)
2. MySqlRow's Debug implementation would be completely different

If this is too ambitious of a change I'm happy to pare it down to just the SqliteRow change as that's fundamentally what I'm after.

[abonander]

This actually shouldn't be considered a breaking change because the Debug output of a type should generally not be considered to be stable. It's right there in the name: it's meant for debugging, not necessarily something to be depended upon in production.

The only time the change of Debug output should be considered a breaking change is if it's explicitly promised to be stable.