Add support for embedded/encoded payloads

[atc0005]

Overview

Add support for embedding an encoded payload within and later extracting the payload from plugin output.

The current Nagios plugin API permits returning human readable information to Nagios for display in a web UI or notifications and metrics for calculations & other data manipulation purposes. As far as I know, Nagios does not permit passing something like a JSON payload directly between the plugin and the monitoring system.

This new (optional) support provides a way for plugins to safely & reliably embed content (e.g., JSON data) within the generated output which can later be extracted by other tools.

Due to current Nagios plugin output length restrictions (8192 bytes), the use of encoded payloads should be used sparingly.

Changes

• add standalone functions for encoding/decoding payload content
• add new Plugin methods for adding (append or overwrite behavior) content to be encoded and embedded as a payload
• add new Plugin methods to override behavior
  • opt out of using delimiters (NOT RECOMMENDED)
  • use custom delimiters
  • set custom payload section label
• minor README updates
  • note new functionality
  • refresh overview & status sections
• add test coverage for new functionality
• add numerous testdata input files
• add basic examples
  • extract and decode an embedded payload from (previous) plugin output
  • add encoded payload

Credit

I consulted ChatGPT when planning the implementation. While later iterations of the code differed significantly, ChatGPT provided code samples that acted as a starting point for the initial encoding/decoding functionality.

Discussions with ChatGPT proved very helpful when deciding between using the base64 encoding format (more common, but less efficient) and the base85 (aka, "Ascii85") encoding format (more efficient). Those discussions also covered concerns such as adhering to the default illegal_macro_output_chars requirements that Nagios imposes for plugin output.

References

• Add support for embedding an encoded payload within and later extracting the payload from plugin output #251
• Test embedding an encoded JSON payload at tail end of LongServiceOutput content check-cert#960