Fix: add a support to pass basic authentication for authentication endpoint

[KoditkarVedant]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #472

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• AuthenticationClientTests.Create_and_revoke_token

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[Copilot]

Pull Request Overview

This PR fixes authentication for the Notion API by adding support for basic authentication in token creation and revocation endpoints, addressing issue #472.

Key changes:

• Add basic authentication parameter support to REST client methods
• Update authentication request classes to include client credentials
• Create helper utilities for basic authentication header generation

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Test/Notion.IntegrationTests/IntegrationTestBase.cs	Refactors environment variable retrieval into a reusable method
Test/Notion.IntegrationTests/AuthenticationClientTests.cs	Adds integration test for token creation and revocation with basic auth
Src/Notion.Client/RestClient/RestClient.cs	Extends REST client to support basic authentication parameters
Src/Notion.Client/RestClient/IRestClient.cs	Updates interface to include basic authentication parameter
Src/Notion.Client/Api/Authentication/RevokeToken/Request/RevokeTokenRequest.cs	Adds client credentials properties for basic auth
Src/Notion.Client/Api/Authentication/RevokeToken/AuthenticationClient.cs	Updates method to pass basic auth parameters
Src/Notion.Client/Api/Authentication/IBasicAuthenticationParameters.cs	Defines interface for basic authentication parameters
Src/Notion.Client/Api/Authentication/HeaderHelpers.cs	Provides utility for creating basic auth header values
Src/Notion.Client/Api/Authentication/CreateToken/Request/CreateTokenRequest.cs	Adds client credentials properties for basic auth
Src/Notion.Client/Api/Authentication/CreateToken/AuthenticationClient.cs	Updates method to pass basic auth parameters

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