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.lock

@@ -136,6 +136,19 @@ version = "1.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "55248b47b0caf0546f7988906588779981c43bb1bc9d0c44087278f80cdb44ba"
 
+[[package]]
+name = "bigdecimal"
+version = "0.4.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1a22f228ab7a1b23027ccc6c350b72868017af7ea8356fbdf19f8d991c690013"
+dependencies = [
+ "autocfg",
+ "libm",
+ "num-bigint",
+ "num-integer",
+ "num-traits",
+]
+
 [[package]]
 name = "bitflags"
 version = "1.3.2"
@@ -1021,6 +1034,16 @@ dependencies = [
  "unicode-segmentation",
 ]
 
+[[package]]
+name = "num-bigint"
+version = "0.4.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a5e44f723f1133c9deac646763579fdb3ac745e418f2a7af9cd0c431da1f20b9"
+dependencies = [
+ "num-integer",
+ "num-traits",
+]
+
 [[package]]
 name = "num-bigint-dig"
 version = "0.8.4"
@@ -1515,6 +1538,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ee6798b1838b6a0f69c007c133b8df5866302197e404e8b6ee8ed3e3a5e68dc6"
 dependencies = [
  "base64",
+ "bigdecimal",
  "bytes",
  "chrono",
  "crc",
@@ -1589,6 +1613,7 @@ checksum = "aa003f0038df784eb8fecbbac13affe3da23b45194bd57dba231c8f48199c526"
 dependencies = [
  "atoi",
  "base64",
+ "bigdecimal",
  "bitflags 2.9.1",
  "byteorder",
  "bytes",
@@ -1632,6 +1657,7 @@ checksum = "db58fcd5a53cf07c184b154801ff91347e4c30d17a3562a635ff028ad5deda46"
 dependencies = [
  "atoi",
  "base64",
+ "bigdecimal",
  "bitflags 2.9.1",
  "byteorder",
  "chrono",
@@ -1649,6 +1675,7 @@ dependencies = [
  "log",
  "md-5",
  "memchr",
+ "num-bigint",
  "once_cell",
  "rand 0.8.5",
  "serde",