feat: add tiktoken as explicit dependency and document Rust requirement

- Add tiktoken>=0.8.0 as explicit dependency to ensure pre-built wheels are used
- Document Rust compiler requirement as fallback in README.md
- Addresses issue #1824 tiktoken build failure

Co-Authored-By: Joe Moura <joao@crewai.com>

pyproject.toml

@@ -30,6 +30,7 @@ dependencies = [
     "pdfplumber>=0.11.4",
     "openpyxl>=3.1.5",
     "blinker>=1.9.0",
+    "tiktoken>=0.8.0",
 ]
 
 [project.urls]