Store Permify CLI credentials by profile

[mdp28]

/claim #2

Summary

• Adds profile-aware credential storage at ~/.permify/credentials (Windows: %USERPROFILE%\.permify\credentials) for endpoint, bearer token, client cert path, and client cert key path.
• Keeps tenant/profile data in the normal CLI config while keeping credentials out of that file.
• Updates configure to collect and persist connection credentials, then validates the connection with those values.
• Updates command clients to create gRPC clients from the active profile, including bearer metadata, TLS endpoints, and optional mTLS client cert/key support.
• Adds focused tests for credential persistence/loading, legacy field compatibility, config secret separation, endpoint parsing, token metadata, TLS/mTLS option setup, and empty endpoint validation.

Verification

• git diff --check passed.
• Local Go toolchain was unavailable on this Windows environment, and repeated portable Go downloads from go.dev reset before completion, so I could not run gofmt/go test ./... locally here.

Notes

• This follows the maintainer guidance in Endpoint, Token, Cert Path, and Cert Key Storage for CLI Tool #2 to use ~/.permify/credentials / C:\Users\USERNAME\.permify\credentials for endpoint, token, certificate path, and certificate key.
• The credential file is written with 0600 mode on Unix-like systems and stored per profile so existing --profile workflows remain supported.