Expose SGCluster flag to use delta (incremental) backups

StackGres relies on WAL-G for performing backups. WAL-G supports incremental (delta) backups, which can be configured by seetting the settings WALG_DELTA_MAX_STEPS and WAL_DELTA_ORIGIN (see docs).

The goal of this feature request is to add under the backups section of the SGCluster configuration to enable it (it is disabled by default).

It is an important feature as it can save a huge amount of space on large databases where changes are small relative to the total cluster size.

Acceptance criteria:

• add under the backups section of the SGCluster configuration to enable it