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.

Config before

model_list:
  - model_name: my-gitlab-model
    litellm_params:
      model: gitlab/bedrock/converse/openai.gpt-oss-120b-1:0
      prompt_id: "hello2"

Config after this change

model_list:
  - model_name: my-gitlab-model
    litellm_params:
      model: gitlab/bedrock/converse/openai.gpt-oss-120b-1:0
      prompt_id: "gitlab::hello2"

You can represent subdirectories as

model_list:
  - model_name: my-gitlab-model_in_sub_dir
    litellm_params:
      model: gitlab/bedrock/converse/openai.gpt-oss-120b-1:0
      prompt_id: "gitlab::folder1::hello2"

Where hello2.prompt is the file in folder.

Relevant issues

#15169

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Type

🆕 New Feature

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[deepanshululla]

@krrishdholakia could you also please review this. thanks.

[ishaan-jaff]

LGTM