Gigachat 3 tool parser and tests

[Mishusha]

This PR adds a tool parser for parsing the function calls made by the Gigachat 3 models.
Models can generate only one function call using format:

{text}<|message_sep|>\n\nfunction call<|role_sep|>\n{json}
Where <|message_sep|>\n\n and <|role_sep|>\n are special tokens.