Add feature fake data provider

[abhinavs1920]

PR Description

Implemented Fake Data Providers for API Dash to streamline testing.
Users can now use dynamic placeholders like {{$randomEmail}}, {{$randomUsername}}, {{$randomId}} in their API requests, which will automatically be replaced by randomized data at runtime.

This feature reduces manual effort during testing and enhances the developer experience.
It builds on the existing environment variable system without changing or breaking existing functionality.
A new section titled Fake Data Providers has been added to the environment management interface to guide users.

Some parts of code are generated with the help of AI tools.

Related Issues

• Closes Implement Mock Data provider for API Testing #496

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• Yes
• No, and this is why: Will add the tests once the approach is finalized.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

[abhinavs1920]

@animator
Please look into this PR.
Thanks!

[abhinavs1920]

@animator @ashitaprasad @DenserMeerkat
please check it, whenever you are available.
Thanks!