feat(sdk+cli): add Credential Kind SDK client and acpctl credential subcommands (RHOAIENG-55817)

- Add types/credential.go: Credential, CredentialList, CredentialTokenResponse, CredentialBuilder, CredentialPatchBuilder
- Add client/credential_api.go: CredentialAPI with Create, Get, List, Update, Delete, GetToken, ListAll
- Add cmd/acpctl/credential/cmd.go: list, get, create, update, delete, token subcommands following agent pattern
- Register credential command in acpctl main.go

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: Ambient Code Bot