add base64 support to multipart parser

[charleystran]

Description of change

This change allows the benefits intake api to allow for the file contents to be base64 encoded on upload to S3

Testing done

rspec and local tests

Testing planned

testing on staging

Acceptance Criteria (Definition of Done)

• a base64 encoded pdf can be uploaded and processed properly

Applies to all PRs

• Appropriate logging
• Swagger docs have been updated, if applicable
• Part of https://github.com/department-of-veterans-affairs/vets-contrib/issues/1990
• Does not contain any sensitive information (i.e. PII/credentials/internal URLs/etc., in logging, hardcoded, or in specs)
• Provide which alerts would indicate a problem with this functionality (if applicable)

[bastosmichael]

LGTM