Add Jinja template support

[ochafik]

Subset of #9639 with just the Jinja templating support (proper tool support to come as follow ups).

• Copies minja.hpp & chat-template.hpp from google/minja at this commit
• Adds --jinja and --chat-template-file flags to llama-server
• Loads tokenizer.chat_template (or tokenizer.chat_template.tool_use if defined, only when the request has tools), and defaults to a simple chatml template.

TODO:

• Add some instructions here
• Add more server tests to exercise the template overrides.
• Add cross-testing in test-chat-template.cpp (note that minja is tested against a lot of templates in its own repo)