Werk #10975: Support local files in Agent Bakery

The agent bakery now recognizes files placed under the local hierachy of the Checkmk site and packages them as a replacement for the corresponding non-local/builtin-files.

Previously, this feature was available for some files like "custom-files" or the agent itself. Now this holds true for all files that get packaged into the baked agent package.

For example, if you want to replace a plugin file, that is located at ~/share/check_mk/agents/plugins/my_plugin, you would place your own version at ~/local/share/check_mk/agents/plugins/my_plugin.

Additionally, the agent bakery watches all files that are relevant for a package and will invalidate the current package upon changes in file size, modification date, or file permissions, resulting in a newly baked package with new agent hash on bake request. This is visible by an orange-colored "Bake agents" button at the agent bakery.

Especially, if a file is added to the local hierarchy as a replacement for a builtin-file, this will be recognized as a file change and result in a new package on bake request.

Please note: Some files will get renamed by the bake process, e.g. the source file check_mk_agent.linux will result in check_mk_agent in the final agent package. In order to get a local file packaged, you always have to place your local file using the source name (Here: ~/local/share/check_mk/agents/check_mk_agent.linux).

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