feat: Add support for primary key for external tables.

[parkma99]

Which issue does this PR close?

Closes #7154

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[parkma99]

Hello @mustafasrepo , I don't know how to add Constraints in protobuf, Could you have some time to help me ? Thanks you!!!

[mustafasrepo]

Hello @mustafasrepo , I don't know how to add Constraints in protobuf, Could you have some time to help me ? Thanks you!!!

Thanks for working on this issue. I have made a implementation that adds support for protobuf in here on top of your work. You can see the last commit. Basically, In short you have to update datafusion.proto file to add new protobuf types (with its own syntax). Once this file is updated, you have to run bash regen.sh script to automatically generate rust code. All the files under datafusion/proto/src/generated/ are automatically generated with this script. You shouldn't touch these files yourself. However, to add this support I had to make some structs under datafusion_common public. If you can come up with a solution where those structs doesn't have to be public. It would be very nice.

[parkma99]

Thanks @mustafasrepo