Add support for random delay in ra-data-fakerest

[dreglad]

Problem

Previously, ra-data-fakerest only supported a fixed delay (in milliseconds). This made it difficult to simulate more realistic, variable network conditions during development or end-to-end testing.

Solution

This PR enhances the delay parameter of the fakeDataProvider to support random ranges:

type Delay = number | boolean | { min?: number; max?: number };

number: Remains supported for fixed delays (backward compatible).
boolean: Passing true enables a default random delay between 500ms and 1500ms.
object: Passing { min: number, max: number } allows setting a custom random range.

How To Test

Added a parameterized test to packages/ra-data-fakerest/src/index.spec.ts to ensure all scenarios are covered with no breaking changes.

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
  • Not a bugfix, but a backwards-compatible new feature.
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
  • Not applicable for this data provider change; behavior is fully verified via unit tests.
• The documentation is up to date
  • Updated README.md with examples

[fzaninotto]

Great! Would you mind changing the PR target to next instead of master? master is only for bug fixes, new features come to next.

[fzaninotto]

i rebased your PR on next