Flattening in Row derive macro + documentation of attributes

[cpg314]

Following issue #30, this implements the serde-like flatten attribute on the Row derive macro, allowing to compose rows as follows:

#[derive(klickhouse::Row)]
struct User {
  age: u8,
  name: string
}
#[derive(klickhouse::Row)]
struct Row {
  #[klickhouse(flatten)]
  user: User,
  credits: u32
}
let users: Vec<Row> = ch.query_collect("SELECT age, name, credits FROM ...").await?;

The commit also adds documentation on the derive macro, in particular::

• The supported serde- and clickhouse-specific attributes
• The issue on field ordering for type hints Type hinting in serialization depends on field ordering #34

This is stacked on PRs from #33 and #33 (tests and documentation). Only the last commit d58c633 is new. See only new changes.

[Protryon]

@cpg314 Once this is rebased and merged, would you like me to cut a release now or wait for #34 ?