Add support for Gitlab multipart

Feature Request

Why

There is no way in current BigBang to set the multi-part chunk size for Gitlab. It is a setting that lives in the backups secret, but it is not templated in the BigBang secret-objectstore resource.

Proposed Solution

Allow to add the multipart_chunk_size_mb = {{ .Values.addons.gitlab.objectStorage.multipartChunkSizeMb }} field to this secret: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/1.8.0/chart/templates/gitlab/secret-objectstore.yaml#L40-45