File-based exports should be organized by subdirectory

[lmsurpre]

Is your feature request related to a problem? Please describe.
While reviewing #1055, I noticed that the files created in a file-based export are placed directly in the configured fileBase directory with a filename that matches the random string that was generated for that particular export.

For systems that have processed a lot of different exports, I think we would benefit from having each export end up in its own subdirectory under the configured fileBase directory instead.

Describe the solution you'd like
Instead of export files like this:

• /JawJfHsEuJFVVD_L9X9roS-0Ewzb2-F8Kx6-FJRmoT0_Condition_1.ndjson
• /JawJfHsEuJFVVD_L9X9roS-0Ewzb2-F8Kx6-FJRmoT0_Observation_1.ndjson
• /JawJfHsEuJFVVD_L9X9roS-0Ewzb2-F8Kx6-FJRmoT0_Patient_1.ndjson
• /oUpzVYJl-ICco8-uKEOGMf4wkqeFGKxxpCK3KY0H744_Condition_1.ndjson
• /oUpzVYJl-ICco8-uKEOGMf4wkqeFGKxxpCK3KY0H744_Observation_1.ndjson
• /oUpzVYJl-ICco8-uKEOGMf4wkqeFGKxxpCK3KY0H744_Patient_1.ndjson

generate output like this:

• /JawJfHsEuJFVVD_L9X9roS-0Ewzb2-F8Kx6-FJRmoT0/Condition_1.ndjson
• /JawJfHsEuJFVVD_L9X9roS-0Ewzb2-F8Kx6-FJRmoT0/Observation_1.ndjson
• /JawJfHsEuJFVVD_L9X9roS-0Ewzb2-F8Kx6-FJRmoT0/Patient_1.ndjson
• /oUpzVYJl-ICco8-uKEOGMf4wkqeFGKxxpCK3KY0H744/Condition_1.ndjson
• /oUpzVYJl-ICco8-uKEOGMf4wkqeFGKxxpCK3KY0H744/Observation_1.ndjson
• /oUpzVYJl-ICco8-uKEOGMf4wkqeFGKxxpCK3KY0H744/Patient_1.ndjson

Describe alternatives you've considered
Continue placing the files directly under the single output directory.

Acceptance Criteria
1.
GIVEN [a precondition]
AND [another precondition]
WHEN [test step]
AND [test step]
THEN [verification step]
AND [verification step]

Additional context
Add any other context or screenshots about the feature request here.

[lmsurpre]

I confirmed this one locally. Exports are now added to their own folder under the configured fileBase.