doc(data): support for auto increment (serial) fields

[p5quared]

Description of changes:

We now support modeling SQL generated auto increment (serial) fields.

e.g.

Database model

-- PostgreSQL 
create table example (
  serialPK SERIAL PRIMARY KEY,
  serialNumber SERIAL,
  content text
);

Amplify model

// Simplification - removed boilerplate
a.schema({
    "example": a.model({
        serialPK: a.integer().default(),
        serialNumber: a.integer().default(),
        content: a.string(),
    }).identifier([
        "serialNumber"
    ]),
})

Notes:

• We only support PostgreSQL
• Only supported field is SERIAL (aka auto increment)
• The special syntax a.integer().default() denotes a generated integer on TS schema
• The special syntax f: Int! @default now exists on GQL schema
• As far as the schema knows, these fields are nullable hence why .identifier now can accept nullable fields. However, new checks block users from improperly specifying a nullable identifier.

Related GitHub issue #, if available:

aws-amplify/amplify-category-api#2883

Instructions

Pending some release propagations before npx ampx generate-schema-from-database will generate the proper field types for SERIAL fields (a.integer().default()).

If this PR should not be merged upon approval for any reason, please submit as a DRAFT

Which product(s) are affected by this PR (if applicable)?

• amplify-cli
• amplify-ui
• amplify-studio
• amplify-hosting
• amplify-libraries

Which platform(s) are affected by this PR (if applicable)?

• JS
• Swift
• Android
• Flutter
• React Native

Please add the product(s)/platform(s) affected to the PR title

Checks

• Does this PR conform to the styleguide?

• Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

• Are any files being deleted with this PR? If so, have the needed redirects been created?

• Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  
  ref: MDX: [link](https://docs.amplify.aws/)
  HTML: <a href="https://docs.amplify.aws/">link</a>

When this PR is ready to merge, please check the box below

• Ready to merge

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[palpatim]

Do we need to specify that .default() can only be applied to .integer() fields?

[p5quared]

Reworded to make integer fields more explicit

If you're working with a Postgres datasource, .default() can be applied to integer fields to represent serial fields.

[josefaidt]

does .default() require an argument here similar to the graphql directive?

[p5quared]

Actually we've removed the required arg for both. .default() maps to @default. There are checks in place so that this special (no argument) case can only be applied if:

1. Datsource is Postgres
2. Fieldtype is integer