Add message transformer interface

[jfzunigac]

Introducing a message transformer interface for LogFileReaders, enabling the transformation of messages before they are forwarded to LogStreamProcessors. The desired transformer along its configuration can be defined within a LogStream configuration file as follows:

# Configuration for transformer
transformer.type=regex_based_modifier
transformer.regex_based_modifier.regex=(?s)^(.+?) (INFO|ERROR|OFF|DEBUG|SEVERE|WARN) (null) (.*)$
transformer.regex_based_modifier.modifiedMessageFormat={log_level: $2, message: $4, timestamp: $1}
transformer.regex_based_modifier.appendNewLine=true

Please note that configuring message transformers is optional, and only one transformer can be applied per log stream. Currently, we are introducing a single transformer: RegexBasedModifier. This transformer uses a regular expression to extract fields from a message and reconstructs it based on the modifiedMessageFormat specification. Additionally, transformers are intentionally disabled for ThriftReaders, as altering Thrift messages would be too expensive.

For example, with the configuration shown above, the following message:

2024-11-15 23:59:37 OFF null 7301524e-6de0-4b01-87b8

would be transformed to:

{log_level: OFF, message: 7301524e-6de0-4b01-87b8, timestamp: 2024-11-15 23:59:37}