chore: add logs for virus scanning duration

[kathleenkhy]

Problem

Currently we do not keep track of how long it will take for files to be downloaded from S3 - this arose from an alarm due to a spike in time taken for scanning and downloading clean attachments

Solution

Added logs in src/app/modules/submission/submission.service.ts to track the start and end of the download from S3

Breaking Changes

• No - this PR is backwards compatible

Manual tests

• Create a form
• Complete a submission of the form
• In datadog, search for "Successfully downloaded file from S3"
• Copy the Clean File Key
• Search for the clean file key in datadog or cloudwatch and check that there will be 2 logs with the messages of "File download from S3 has started" and "Successfully downloaded file from S3".

Datadog:

Cloudwatch:

Searching by clean file ID of form submitted:

[KenLSM]

LGTM!