feat: add DestinationHttpClient to simplify calls to target systems

[jplbrun]

Description

Adds DestinationHttpClient, a requests wrapper that makes calling a target system from a destination straightforward. Previously, users had to manually fetch an auth token from destination properties and build the Authorization header themselves, and also manually set the sap-client header for OnPremise destinations. With this change:

dest = client.get_destination("my-destination")
http = DestinationHttpClient(dest)
response = http.get("/api/resource")

The client pre-bakes auth headers from dest.auth_tokens (populated by the v2 API) and SAP ERP headers (sap-client, sap-language) into the session at construction time.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Get a destination using the v2 API: dest = client.get_destination("my-destination")
2. Instantiate the client: http = DestinationHttpClient(dest)
3. Call the target system: response = http.get("/api/resource")
4. Verify that headers are sent automatically without manual setup
5. Run unit tests: uv run pytest tests/destination/unit/test_destination_http_client.py -v

Related Issue

Closes #35

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages