Persist token/cert config for CLI client

[TheDerbiedOne]

Fixes #2

What

Adds persistent credential/config storage via the existing CLI config YAML:

• endpoint
• bearer token (optional)
• TLS client cert path (optional)
• TLS client cert key path (optional)

How

• Extends core/config.CoreConfig YAML fields for token, cert_path, cert_key.
• Updates permctl configure to prompt for token/cert paths and persist them.
• Updates core/client.New to:
  • use TLS when endpoint is https://
  • optionally attach authorization: Bearer <token> using grpc.WithPerRPCCredentials (TLS-only)
  • optionally present a client cert (mTLS) when both cert path + key are provided

Safety

• Token is only sent over TLS; non-https:// endpoints error at client creation.
• No credentials embedded in code; config schema + client wiring only.

[TheDerbiedOne]

Update: adjusted implementation to follow maintainer note — connection credentials (endpoint/token/cert paths) are now stored in (0600), and the client loads from that file. Main profile YAML no longer persists token/cert paths.

[TheDerbiedOne]

Follow-up: credentials file path is ~/.permify/credentials (written with 0600 perms). Sorry the prior comment dropped the path due to shell quoting.