feat: add get file apis

[code-monad]

This pull request introduces new functionality for retrieving and saving file content from the blockchain, along with some minor updates to the package.json and src/index.ts files. The main changes include the addition of utility functions and the necessary updates to import and export statements.

New functionality:

• Added getFileContentFromChain function to retrieve complete file content from the blockchain by following backlinks.
• Added saveFileFromChain function to save file content retrieved from the blockchain to disk.
• Introduced safelyDecode utility function to safely decode buffer to string, handling errors gracefully.

Updates to import/export statements:

• Updated src/index.ts to include new imports for getFileContentFromChain and saveFileFromChain. [1] [2]

Minor changes:

• Added a new script example:retrieve in package.json to run the new functionality examples.