Simplify API and use "state" to improve type-safety

[connec]

The upshot of this PR is that:

• Tx is the only type that needs to be handled directly (i.e. used or otherwise named).
• Tx's FromRequestParts implementation requires axum_sqlx_tx::State, which goes someway to making it harder to forget to supply the extractor's dependencies (it doesn't prevent compilation when Layer is forgotten, but the setup methods return (State, Layer) making it harder to miss).

---

• 0927aab chore: doc typo

• c9eccaf feat!: introduce State to make Tx more type-safe

  Tx now implements FromRequestParts<State>, meaning the Router must
  be provided an instance of State in order for it to compile.

  State can be constructed with Layer::new, which now returns a tuple
  of Layer and State. The idea behind constructing both in the same
  function is to make it harder to forget to add the Layer, however
  Layer::new is perhaps not the best place for it long-term.

  Ultimately this has the desired effect of making Tx more type-safe -
  applications that attempt to use Tx without providing State won't
  compile, and the API for obtaining State makes it harder to forget to
  add the Layer.

  BREAKING CHANGE: Layer::new now returns a (Layer, State) tuple. This
  can be consumed easily by destructuring assignment. Tx now requires
  State to be provided on Routers in order for the Router to be
  usable.

• 32a0a0f feat: introduce Tx::setup, Tx::config, and Config APIs

  This will centralise the configuration API and reduce the number of
  types that need to interacted with directly (i.e. used) down to just
  Tx.

• e8f694e refactor!: remove Layer::new and Layer::new_with_error

  This leaves Tx::{setup,config} as the only entrypoints to the API.

  Error handling documentation was also rewritten.

  BREAKING CHANGE: Layer::{new,new_with_error} have been removed. Use
  Tx::{setup,config} instead.

• e2701a7 refactor!: rationalise generic error type bounds

  Required bounds are now present on more impl blocks to provide better
  diagnostics if error types are missing required traits.

  The Layer error type is now only required to implement
  From<sqlx::Error> (technically sqlx::Error is required to implement
  Into<LayerError> since this is more flexible).

  BREAKING CHANGE: The tighter bounds on impl blocks shouldn't break
  already working code, however the change to the Layer error type
  bounds will break error overrides that implement
  From<axum_sqlx_tx::Error> - this must be changed to
  From<sqlx::Error>.

• e896b5a chore: doc fix