[flagd-provider] consistent configuration options / add certPath support

[toddbaert]

We should make our options consistent, including the env vars. I think we want:

| Option name | Environment variable name | Type    | Default   |
| ----------- | ------------------------- | ------- | --------- |
| host        | FLAGD_HOST                | string  | localhost |
| port        | FLAGD_PORT                | number  | 8013      |
| tls         | FLAGD_TLS                 | boolean | false     |
| socketPath  | FLAGD_SOCKET_PATH         | string  | -         |
| certPath    | FLAGD_SERVER_CERT_PATH    | string  | -         |

We have all of these except FLAGD_SERVER_CERT_PATH.