feat(decorators): add @AtLeastOneField class-level decorator

[francoabottaro]

Related issue

Closes #2676

What does this PR do?

Adds @AtLeastOneField, a class-level decorator that validates at least
one field of a DTO has a non-empty value. Useful for PATCH endpoints
where all fields are optional but the body cannot be empty.

Example Usage

@AtLeastOneField()

class UpdateUserDto extends PartialType(CreateUserDto) {}

Changes

• feat: src/decorator/object/AtLeastOneField.ts
• test: src/decorator/object/AtLeastOneField.spec.ts
• export: src/decorator/object.ts

[francoabottaro]

Hi! I've opened a PR for this feature: #2680

It includes:

• The @AtLeastOneField decorator implementation
• Unit tests
• Export in src/object/decorators.ts

@braaar