feat(agent): split legacy username/password login into its own file

[roeezis]

Summary

Part 2 of the SMS/Conjur JWT authentication series (split out of #817 for reviewability). Stacked on #818 — includes #818's commit until that merges, so the diff here will shrink to just this PR's own change once #818 lands.

identity.go mixed shared client/token-cache plumbing with the CyberArk Identity username/password (UP) login flow. Moves the UP-specific code into username_password.go so the shared plumbing stays easy to find once a second login mechanism (Conjur JWT) is added alongside it.

No behavior change — pure extraction, plus unexporting ActionAnswer and exporting the mock's success credentials for other packages' tests.

Test plan

• go build ./...
• go test ./internal/cyberark/identity/...

[mladen-rusev-cyberark]

Verified the "no behaviour change" claim mechanically — sorting all lines of the pre-image identity.go against the sorted union of the post-image identity.go + username_password.go yields only comment/import differences, no code changes. The split is a good idea and lands cleanly.

One correction needed (inline), plus: the Python SDK reference link dropped from the Client doc comment is the only pointer in the repo to the reference implementation this client mirrors — worth moving to username_password.go rather than deleting.

Reviewed as this PR's own commit (2483110) rather than the cumulative diff against master.

[roeezis]

Fixed — deleted the duplicate actionAnswer const, mock.go now uses the single exported ActionAnswer. Also corrected the commit message.