feat: add support for Proto Columns to Connection API

[harshachinta]

Adds support for Proto Columns feature in the Connection API. This is needed to support Proto Columns in the JDBC driver PR-1252.

[gauravpurohit06]

We supporting reading from the file here? If Yes where should file be located just on resources or local path ?
We had similar discussion on #2277 (comment).

[harshachinta]

Yes we are supporting reading from a file path given through a client side statement. This is needed to support ORM's and other frameworks. This discussion is available in the design doc.
Just a small brief, in PR#2277, we were treating file as a resource which brings in complications as they rely on class path. However, in this method we instead used File system to read from relative or absolute path.

[rajatbhatta]

Let's also add a NonNull annotation to the argument here. Similarly in other places wherever a null value is not expected.

[harshachinta]

These convert methods are not used by customers and is getting called from link. We generally use this annotation to warn users when they pass null. But here it is used in code internally and as users don't use this function, I guess it is not needed.

[rajatbhatta]

It is being called from the same package and not meant to be public to customers. Then, should we make the methods package-protected?

[olavloite]

These methods cannot be package-private, because they implement a method from an interface. The class itself is package-private, which protects the class and all its methods from being used by 'outside' users.

[rajatbhatta]

Suggested change

	* @return The proto descriptor that will be used with the next DDL statement (single or batch)
	* @return The proto descriptor that will be used only with the next DDL statement (single or batch)

[olavloite]

These methods cannot be package-private, because they implement a method from an interface. The class itself is package-private, which protects the class and all its methods from being used by 'outside' users.