[Feature] Add endpoint to return a list of available plugins.

[YourTechBud]

The problem faced currently?

There is no way to figure out how many plugins are available/ registered in SC.

How can we solve it?

• Each Source which feeds into a plugin (eg. OPAPolicySource) should implement a source.Plugin interface.
• The interface will be defined by the source manager.
• The Plugin interface will have just one method, GetPluginDetails() which returns a struct containing name, driver and schema.
  • Name will be the name of the source
  • Driver will be the handler which needs to be initialised for that plugin (for OPAPolicySource, the driver will be opa).
  • Params will always be nil for now.
• The Source manager should expose an api /sc/v1/plugins which returns array of plugins registered.

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