Add GitlabPromptCache and enable subfolder access

[deepanshululla]

Title

breaks the pr into smaller pieces
#15170

This change doesn't do anything much by itself but lays the foundation for a followup PR.

GitLabPromptCache is a container where all prompts in the gitlab repo will be cached in memory.

To make it work with GET {{url}}/prompts/prompt_id endpoint changing gitlab folder paths to use :: instead.

Relevant issues

#15169

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Type

🆕 New Feature

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