feat: add IsMutuallyExclusiveWith decorator

[Abdalrahman-Amr-Dev]

Description

Adds a new @IsMutuallyExclusiveWith(property: string) decorator that checks
a property and a related property are not both provided at the same time.

This is useful for DTOs/models where exactly one of two mutually exclusive
fields should be set — e.g. "provide either emailAddress or phoneNumber,
not both".

Example

```typescript
class ContactDto {
@IsMutuallyExclusiveWith('phoneNumber')
emailAddress?: string;

phoneNumber?: string;
}
```

Validation fails only when both emailAddress and phoneNumber are
present (not undefined/null). It passes when either one, or neither, is set.

Changes

• Added src/decorator/common/IsMutuallyExclusiveWith.ts
• Exported the new decorator/function from src/decorator/decorators.ts
• Added tests in test/functional/validation-functions-and-decorators.spec.ts
• Documented the decorator in README.md

Checklist

• npm test passes
• npm run lint:check passes
• npm run prettier:check passes
• Docs updated (README)