Add custom_name config for filename

[ron-damon]

Description of change

This PR adds an optional option custom_name to specify a custom name instead of the stream name in the resulting filename.
E.g: Stream name is entities and timestamp is 20210218T000000

1. custom_name="" and do_timestamp_file=true

{
    "destination_path": "/tmp",
    "custom_name": "",
    "do_timestamp_file": true
}

Results in: /tmp/entities-20210218T000000.jsonl

2. custom_name="entity" and do_timestamp_file=true

{
    "destination_path": "/tmp",
    "custom_name": "entity",
    "do_timestamp_file": true
}

Results in: /tmp/entity-20210218T000000.jsonl

3. custom_name="entity" and do_timestamp_file=false

{
    "destination_path": "/tmp",
    "custom_name": "entity",
    "do_timestamp_file": false
}

Results in: /tmp/entity.jsonl

3. custom_name="" and do_timestamp_file=false

{
    "destination_path": "/tmp",
    "custom_name": "",
    "do_timestamp_file": false
}

Results in: /tmp/entities.jsonl

Manual QA steps

• All combinations of destination_path, custom_name and `do_timestamp_file tested and worked as expected

Risks

• None; this is an optional configuration, new implementations will not break

Rollback steps

• Revert this branch

[andyhuynh3]

@ron-damon LGTM (sorry I'm a year late -- just realized there was a PR created). I'll go ahead and merge this in.