Accept current_file for chat requests

What does this MR do and why?

Adds optional current_file parameter which will be used for passing additional context about source file being worked with (gitlab-vscode-extension#1084 (closed))

current_file is only added to GitlabContext, but not used yet in the code - will be used in the next step by tools which support slash commands (POC - !135578 (diffs)).

Related to #429911 (closed)

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