[mssql] support for uuid

I'm using f345c23e51717f855d449280ac79d6a65bf71997

I'm using the `query!` macro.

toml
```
[dependencies]
sqlx = { git = "https://github.com/launchbadge/sqlx", features = [ "chrono", "mssql", "uuid" ] }
```

```
^ the trait `sqlx_core::encode::Encode<'_, sqlx_core::mssql::database::Mssql>` is not implemented for `uuid::Uuid`
```