Add config file class for C++ Conan `conanfile.txt`

What does this MR do and why?

Background

The Repository X-Ray is a tool that searches for dependency manager configuration files (aka "lock files") and then extracts a list of libraries from the content.

In #474306 (comment 2025085630), we decided to migrate the Repository X-Ray functionality into the GitLab Rails monolith. This gives us two main benefits: (i) it will eventually allow us to run the service outside of the CI pipeline, and (ii) we can maintain the parsing logic centrally so that other domains can utilize it.

Base MR

In !162313 (merged), we introduced the ConfigFiles::Base class where the intention is for each dependency manager config file type to be represented by a child class. The child class contains the parsing logic to extract a list of libraries and their versions from the file content.

This MR

In this MR, we introduce the CppConanTxt config file class that parses conanfile.txt.

For references to the existing parsing logic in the Repository X-Ray repo, see:

• https://gitlab.com/gitlab-org/code-creation/repository-x-ray/-/blob/6634564818dffdeac938014a190b0c604b1da6e0/internal/scanner/scanner.go#L399
• https://gitlab.com/gitlab-org/code-creation/repository-x-ray/-/blob/6634564818dffdeac938014a190b0c604b1da6e0/internal/deps/deps.go#L185

Resolves part of #476177 (closed).

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Related to #476177 (closed)