SymLinkName, PidFileName, and AgentPrefix support logtail_mode.

[quzard]

This pull request introduces several new utility methods and constants to the AppConfig class in the core/app_config module. These changes aim to enhance the configuration management and file naming conventions based on the logtail_mode flag.

Key changes include:

New utility methods in AppConfig:

• Added GetSymLinkName method to determine the symbolic link name based on the logtail_mode flag.
• Added GetPidFileName method to generate the PID file name depending on the logtail_mode flag.
• Added GetAgentPrefix method to return the appropriate prefix for the agent based on the logtail_mode flag.

Header file updates:

• Declared the new utility methods (GetSymLinkName, GetPidFileName, and GetAgentPrefix) in core/app_config/AppConfig.h.

Constants definition:

• Introduced LOONGCOLLECTOR_PREFIX constant in core/common/version.h to support the new methods.

Additional includes:

• Included common/version.h in core/app_config/AppConfig.cpp to access the newly defined constants.