Allow provided schema, schema cache

[lovromazgon]

Description

This PR contains two additions to schemas:

• Allows the connector to return a record with structured data and an attached schema. This means the connector doesn't have to take care of encoding the data, it can only provide the schema and the middleware will use the schema to encode the data.
  • The use case are DB connectors like Postgres, which can do a better job extracting the schema directly from the DB. Those can simply attach the extracted schema to the record and return structured data as always, the middleware will take care of the encoding.
• Adds caches around the standalone schema service.
  • Note that the in-memory schema service is also cached, as it would otherwise not reflect the correct behavior, it's not smart enough to detect the same schema and will always increment the version and ID on CreateSchema. The cache prevents this.

Fixes #158

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same
  update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[hariso]

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