Add sqlx example with migrations (#722)

* Add sqlx example with migrations

Simple use case for sqlx based on tokio postgres example.
Sqlite database is created on execution in ./target directory
and migrations are then run against it.

* sqlx example uses postgres instead of sqlite3

Also removed migrations and database creation code.

examples/sqlx-postgres/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "example-sqlx-postgres"
+version = "0.1.0"
+edition = "2018"
+publish = false
+
+[dependencies]
+axum = { path = "../../axum" }
+tokio = { version = "1.0", features = ["full"] }
+tracing = "0.1"
+tracing-subscriber = { version="0.3", features = ["env-filter"] }
+
+
+sqlx = { version = "0.5.10", features = ["runtime-tokio-rustls", "any", "postgres"] }