Add More API Endpoints

[nasbench]

This PR adds a couple of API endpoints to make it easy to integrate sigconverter into projects.

• /getbackends endpoint to list available backends
• /getformats endpoint to list available formats per backend
• /getpipelineallowedbackend endpoint to list the allowed backends for every pipeline

FYI: These will be used by the sigma vscode extensions

[M3NIX]

@nasbench If we intend to introduce API Endpoints I think we should keep it more REST like.

These would be my suggestions for changes:

• renaming the endpoints because if its a GET request we do not need get in the url path aswell. That seems to be redundant for me.
• the API endpoints should have some parameters for filtering the results
• To make clear that these are API endpoints add /api/v1/ in before. That makes it more easy to add breaking changes later by introduction a new version /api/v2/ (if needed)

Here some examples of how I imagine it:

URL	Response
/api/v1/backends	List of all backend with identifier and description (same information like in sigma list targets)
/api/v1/formats?backend=splunk	Return all formats for the splunk backend with name and description (same information like in sigma list formats splunk)
/api/v1/formats	Return all formats for the backends with name and description (same information like in sigma list formats <backendname>) + the backend name for the specific format
/api/v1/pipelines	List of all available pipelines (same information like in sigma list pipelines)
/api/v1/pipelines?backend=splunk	only return pipelines which have all or splunk in the allowed backends list

Let me know what you think

[nasbench]

You went for the more official route :) My idea was just to do a quick API endpoints to get the data.

But no worries, I'll make the changes to make REST like a little bit. Thanks for the suggestion.