(WiP) Enhanced type verification when using custom mappings

[purbon]

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This is a work in progress PR, please don't merge.

This PR introduced a verification process to assure that no unexpected type is used, so problems like the ones described in #342 are not possible.

There is still pending work in the PR before considering it done:

• [x] Add the verification process to the getMapping function
• [ ] Add the verification process to the createMapping function
• [ ] Make this verification configurable and result cached to reduce the amount of calls to Elasticsearch.

Fixes #342

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