turbinia-client support custom turbinia config names

### What steps will reproduce the bug?

1. When running a `turbinia-client` request
2. Setting the environment `TURBINIA_API_CONFIG_PATH` to a different path
3. Command that was run: `turbinia-client submit googleclouddisk --project $GCP_PROJECT --zone $GCP_ZONE --disk_name $DISK --request_id $REQUEST_ID`

### What is the expected behavior?

Turbinia client to parse the config without errors. I feel like we should be able to pass in a file or directory it seems to only accept directories and has to have the name `turbinia_api_config.json`?

### What do you see instead?

```
TURBINIA_API_CONFIG_PATH=/home/wyassine/.turbinia_e2e_config.json
2023-11-30 19:14:22,832 ERROR turbinia - Unable to read the configuration file [Errno 20] Not a directory: '/home/wyassine/.turbinia_e2e_config.json/.turbinia_api_config.json'
```

### Additional information

_No response_

Start with the turbinia-client configuration-loading path and reproduce the command using TURBINIA_API_CONFIG_PATH set to /home/wyassine/.turbinia_e2e_config.json. Check how the path is interpreted when it points to a file rather than a directory. Done means the client parses a custom configuration file path without appending turbinia_api_config.json incorrectly.

Written by the indexing model from the issue text.