feat: Add generated and generated_always attributes

This commit introduces support for PostgreSQL generated columns by
adding two new field attributes to the `Georm` derive macro:
`#[georm(generated)]` and `#[georm(generated_always)]`.

The `#[georm(generated_always)]` attribute is for fields that are
always generated by the database, such as `GENERATED ALWAYS AS
IDENTITY` columns or columns with a `GENERATED ALWAYS AS (expression)
STORED` clause. These fields are now excluded from `INSERT` and
`UPDATE` statements, preventing accidental writes and ensuring data
integrity at compile time.

The `#[georm(generated)]` attribute is for fields that have a default
value generated by the database but can also be manually overridden,
such as `GENERATED BY DEFAULT AS IDENTITY` columns. These fields
behave similarly to `#[georm(defaultable)]` fields, allowing them to
be omitted from `INSERT` statements to use the database-generated
value.

For now, the behaviour is the same between `#[georm(generated)]` and
`#[georm(defaultable)]`, but the addition of the former now will be
useful for future features.

Key changes:
- Added `generated` and `generated_always` attributes to
  `GeormFieldAttributes`.
- Introduced `GeneratedType` enum in the IR to represent the different
  generation strategies.
- Modified the `create` and `update` query generation to exclude
  fields marked with `#[georm(generated_always)]`.
- Integrated `#[georm(generated)]` fields with the existing
  defaultable struct logic.
- Added validation to prevent conflicting attribute usage, namely
  `#[georm(generated)]` and `#[georm(generated_always)]` on the same
  field.

Implements #3

Cargo.toml

@@ -32,7 +32,7 @@ georm-macros = { version = "=0.2.1", path = "georm-macros" }
 [workspace.dependencies.sqlx]
 version = "0.8.6"
 default-features = false
-features = ["postgres", "runtime-tokio", "macros", "migrate"]
+features = ["postgres", "runtime-tokio", "macros", "migrate", "bigdecimal"]
 
 [dependencies]
 sqlx = { workspace = true }