feat: external connectors

[nabokihms]

Signed-off-by: m.nabokikh [email protected]

Overview

This is the very first naive gRPC-based implementation of external connectors support. Two connector types have been added: external password connector and external callback connector.

What this PR does / why we need it

With these changes, it becomes possible to write custom connectors utilizing gRPC API or the SDK for Golang (I have added examples for both types).

Closes #1907

Special notes for your reviewer

• Dex communicates with external connectors only via localhost because of security concerns.

• The external-gitlab example of the callback external connector is a copy-paste of the in-tree gitlab connector just to see how easy it will be to convert old connectors/write new ones. However, I'm highly concerned about merging it to master. It seems a lot better to develop some brand new connector in a separate repository.

Does this PR introduce a user-facing change?

Add external connectors support.

[sagikazarmark]

Thanks @nabokihms for working on this!

Without trying to be the bad guy here: I think we are still lacking clarity on a lot of the details that would be nice to understand before going down this road (or any roads).

I opened a separate issue that vaguely explains what my problem is in #2370

In short: I'd like us to write down everything we know (goals, non-goals, scope, etc) so we can make good decisions.

I'm already working on a document for the middleware thing. Do you think @nabokihms you could come up with something for this one?

Basically, I'd like to understand:

• What functionality do we want to "externalize"?
• How does the process look like?
• Can we make it abstract enough so that the implementation doesn't matter? If not, what are the requirements towards the implementations?
• What are the attack vectors that we might introduce into Dex this way?

It's pretty late here, so we might want to have a discussion at some point instead of me just trying to dump my thoughts in the evening. Hopefully it's somewhat clear what I'd like us to do.