feat: Initial work on CyberArk Identity client

[SgtCoDFish]

This includes a bunch of testing data obtained experimentally from testing against an integration-environment tenant.

I've confirmed that I'm able to log in using this code in some scenarios; more work is needed on this client before it can be used but this is quite a lot of lines added as-is and it's worth getting an initial review of the logic so far.

Most docs are linked in comments, but the most important are:

• StartAuthentication
• AdvanceAuthentication

The open source Identity SDK in Python may also be helpful: https://github.com/cyberark/ark-sdk-python/blob/main/ark_sdk_python/auth/identity/ark_identity.py

[wallrj]

The open source Identity SDK in Python may also be helpful: https://github.com/cyberark/ark-sdk-python/blob/main/ark_sdk_python/auth/identity/ark_identity.py

While digging into that, I discovered a very new Golang SDK (1 month old), written by the same author:

• https://github.com/cyberark/ark-sdk-golang

It seems to have an auth / identity package.
It's not as well tested as this, but shall we join forces with them and contribute our tests to that SDK?

[wallrj]

Looks good to me to merge and build upon in future PRs, unless we decide we can use the new CyberArk SDK: https://github.com/cyberark/ark-sdk-golang

My overall comments are:

• I was a bit confused about the password / secret handling, and I would like to read more about the vision for that.
• The new files should all have copyright / apache license headers, as per the LICENSE file.
• I'd like to see an example of how you run the cmd/testidentity tool.

[wallrj]

I wanted to try this out, but it's not obvious what URL or credentials to use.
Suggest adding an explanatory comment with an example of usage.

[SgtCoDFish]

I can provide credentials in another place - because this PR is public I don't want to share any details of our private test env for this. I'm sure it would be fine to do so, but I'm being super careful about what I share and don't share in public.

[SgtCoDFish]

While digging into that, I discovered a very new Golang SDK (1 month old), written by the same author:

😭 This is the pain of integrating with a new company - nobody I've spoken to so far mentioned this. Definitely that repo will be the longer-term way to go, although I suspect we should merge a version of this here and then iterate from there.

First I'll fix up based on your comments!

[SgtCoDFish]

@wallrj I've updated it now. Also added a retry for the login process, and as part of that I migrated the other use of the backoff library to the latest and greatest version

[SgtCoDFish]

Going to merge this now so we can build on it later!