Add log level configuration and tests for logger functionality

[Nicklas2751]

Closes #447

Summary

This pull request introduces a configurable logging system to libpak, allowing users to control log verbosity using the BP_LOG_LEVEL and BP_DEBUG environment variables. The changes add a new log level hierarchy, update the logger implementation to respect these levels, and provide comprehensive tests and documentation.

Log Level Configuration and Implementation:

• Introduced a LogLevel type with levels (none, error, info, debug) and logic to parse and select the active level based on BP_LOG_LEVEL and BP_DEBUG environment variables. BP_DEBUG takes precedence, and unknown values default to info. (log/logger.go)
• Updated the logger initialization in NewPaketoLoggerWithOptions to enable or disable specific writers based on the configured log level, ensuring only relevant logs are emitted. (log/logger.go)

Testing:

• Added unit and integration tests to verify parsing of log levels, correct environment variable precedence, and logger output at each level. (log/logger_test.go)

Documentation:

• Expanded the README.md with a new section explaining log level configuration, use cases, environment variable precedence, and caveats. (README.md)

Use Cases

Let user control if and how much logging they want. Disable logging is useful for containers running processes or servers in a buildpack container that communicate via stdio like a MCP server. See #447

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Nicklas2751 / name: Nicklas Wiegandt (d6efb23)