feat: add -takeout, -list commands

[dzianisv]

This commit introduces two new commands:

• gocryptfs -takeout CIPHERDIR PATH DESTDIR: Decrypts a specific file or directory from the encrypted filesystem and moves it to a destination, deleting the original encrypted file.
• gocryptfs -list CIPHERDIR: Lists the decrypted file and directory names in a tree-like structure.

These commands enhance the utility of gocryptfs by providing more granular control over file decryption and better visibility into the encrypted filesystem's contents without requiring FUSE.

fix: list command outputs full decrypted file paths

The previous implementation of the list command outputted a tree-like structure and failed to decrypt special files. This commit modifies the list command to:

• Output full, relative decrypted file paths, similar to git ls-files.
• Correctly handle and skip special files like gocryptfs.diriv and gocryptfs.conf.
• Decrypt each component of the path individually to ensure correct decryption of nested directories.

fix: takeout command handles plaintext paths correctly

The takeout command previously failed when provided with a plaintext path, as it attempted to directly access the encrypted equivalent. This commit refactors the takeout command to:

• Walk the entire encrypted CIPHERDIR.
• Decrypt each file's relative path on the fly.
• Compare the decrypted path with the user-provided plaintext PATH.
• Only process files and directories that match or are children of the specified PATH.
• Add a helper function decryptRelativePath to encapsulate the path decryption logic.
• Skip files starting with ._ to avoid decryption errors with macOS metadata files.

fix: takeout command handles longnames and special files

The takeout command previously failed when encountering longnames (hashed filenames) and special files (like macOS metadata files starting with ._). This commit addresses these issues by:

• Adding IsLongName and ReadLongName functions to internal/nametransform/names.go to correctly identify and read the original plaintext names of longname files.
• Modifying take_out.go to use these new functions, ensuring that longnames are properly decrypted.
• Expanding the special file skipping logic in take_out.go to include files starting with ._, preventing decryption errors with macOS metadata.
• Gracefully handling decryption errors for non-gocryptfs files by logging them at a debug level instead of warnings.

fix: takeout and list commands handle non-gocryptfs filenames

The takeout and list commands previously failed when encountering filenames that were not valid gocryptfs encrypted names (e.g., non-encrypted files, macOS metadata, or files copied without gocryptfs's knowledge). This commit addresses these issues by:

• Adding IsValidBase64 function to internal/nametransform/names.go to check if a string is a valid base64 encoding.
• Modifying decryptRelativePath in take_out.go and list.go to use IsValidBase64. If a filename component is not a valid base64 string, it is now treated as a literal plaintext name and passed through without decryption. This prevents "bad message" and "padding too long" errors.
• Ensuring that only properly encrypted filenames are subjected to decryption, making the tools more resilient to unexpected file types.$

[rfjakob]

Hi, uh, first, is this AI generated? Please declare this in the PR already.

These are big features and will need to go big in tests. I don't see a single one right now.

[rfjakob]

Not a complete review. But tests are completely missing. Will continue the review when there's some tests.