Refactor ConnectorTableFunction into interface

[kasiafi]

This change refactors ConnectorTableFunction from abstract class into interface. It also adds an abstract class AbstractConnectorTableFunction, which is meant to be base for all implementations of ConnectorTableFunction.

This is a SPI change.

Related to: #12325 (comment) (an interface is needed to apply a class-loader-safe wrapper)

Docs change included.

# SPI
* Change `ConnectorTableFunction` into an interface and add `AbstractConnectorTableFunction` class as the base implementation of table functions.

[findepi]

this should be final, per https://github.com/trinodb/trino/blob/master/.github/DEVELOPMENT.md#additional-ide-configuration

However, this should also be package-private, we don't want to make this part of SPI's contract.
Since this is used shared between io.trino.spi.ptf and io.trino.spi it cannot be package-private.

Please add this as a package-private utility class in io.trino.spi.ptf package, and use it only there.
Revert io.trino.spi.procedure changes.

[findepi]

This should say that AbstractConnectorTableFunction is helpful.
We shouldn't require extending from the class. The interface is the contract.

When thinking more about this, I am thinking what are these checks that AbstractConnectorTableFunction does. What would happen if a connector implemented the interface directly. Maybe the checks should be moved to TableFunctionRegistry?

[kasiafi]

I think that AbstractConnectorTableFunction is helpful because it provides the constructor.

However, it is probably a must to move the checks to the registry in case someone implements the interface directly.

[findepi]

let's do this. to avoid duplicated logic, let's remove them from AbstractConnectorTableFunction.

[kasiafi]

Yeah, I'm going to leave only rnn there, and do the full validation in the registry.

[kasiafi]

@findepi PTAL