add secure aggregation framework

Description of the MR

Covers issue #352 (closed) aka infrastructure for secure aggregation.

Guidelines for MR review

General:

• give a glance to DoD
• check coding rules and coding style
• check docstrings (eg run tests/docstrings/check_docstrings)

Specific to some cases:

• check if breakpoint is properly updated (breakpoint/load_breakpoint in Experiment(), save_state/load_state in aggregators, strategies, secagg, etc.)