Skeleton for internal indexing APIs

[franckc]

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Description:

This PR implements the following:

• Some code refactoring/cleanup
• Skeleton for two new internal endpoints that can be used by the future Node.js based EventListener for indexing listings in the search back-end and in the database. The APIs are restricted to internal usage by checking an access token. The token could be added to the EventListener config via ENVKEY. Note that the interface and logic for these APIs is very barebone (or empty) - the goal is to have those APIs in place for initial development/integration with the EventListener and to flesh out the details as we make progress.

Here is an example for calling the indexing API: curl -i -X PUT http://localhost:5000/api/indexing/search/listing -H "X-Internal-API-Token: dev-internal-api-token" -H "Content-Type: application/json" -d' { "listing": { "contract_address": "0x123", "title": "nice title", "description": "boo" } } '

[franckc]

@DanielVF When you got a min, do you mind reviewing this ? I'd like to get it in. As mentioned in the comments this is just a starting point and will need many more iterations to integrate with the EventListener... Thanks ! :)