Feature: Database Adapter for PostgreSQL with pgVector

[ppanopticon]

This pull requests adds a new plugin that contains a database adapter for PostgreSQL with pgVector. The adapter has all the functionality provided by the existing PostgreSQL adapter.

In addition, this PR implements the following changes:

• All Values used in Descriptors are now defined by an underlying type system (see Type and Value)
• All StructDescriptor implementations inherit from MapStructDescriptor, which is backed by a HashMap. This leads to a cleaner handling of StructDescriptors in the database layer.
• Adds basic facilities to run unit tests for CI / CD.
• Simplifies existing Cottontail DB database driver.

[ppanopticon]

@lucaro Do we still need the StringConnection and what is its purpose?

[lucaro]

StringConnection was primarily a debug mechanism. The plan was to also use it as a basis for a connection that writes directly to the disc for later import, analogous to the JSONWriter in cineast.

[ppanopticon]

Unless there are other comments or change requests, I will merge this PR by the end of the week (after our vitrivr-engine meeting).