Add project scan service

What does this MR do and why?

内容安全服务优化

提供检查某个仓库中主分支下当前所有文件的方案：

1. 获取仓库默认分支的最新commit

ref = project.default_branch
recent_commit = project.repository.commit(ref)

2. 基于最新commit 递归获取仓库文件树

root_tree = repository.tree(recent_commit.id, "", recursive: true)

3. 依次获取目录下文件最新commit列表

commits = project.repository.list_last_commits_for_tree(
  recent_commit.id,
  File.join(path, ""),
  offset: 0, limit: TREE_COMMITS_LIMIT
)

4. 依次检查文件树内容，把对应内容发送到 CVS 检查

if type == :tree
  tree = repository.tree(commit.id, path)
  tree_validate(commit, tree)
elsif type == :blob
  blob = repository.blob_at(commit.id, path)
  validate_blob(commit, blob)
end

1. 添加 ProjectScanService
2. 添加 ProjectScanWorker
3. 添加 content_blocked_states/project_scan api, 目标用户 SaaS Admin

related to issue: https://jihulab.com/gitlab-cn/internal/content-security-backend/-/issues/36

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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