Move aggregation param definition to plugin metadata

[jmcook1186]

What
We currently expect aggregation method to b e set in aggregation config in the manifest. However, time-sync expects to find those data for every parameter, not just those that are being aggregated. This means we need to incorporate the aggregationMethod into the plugin metadata so it can be handled the same way as units and prevent time sync from falling back to default method uneccessarily.

Plugins should have default values defined in their source code but should be overrideable from the manifest, as per the units.

Why
To prevent time sync from resorting to fallback aggregation methods and emitting lots of warnings.

Context
Part of the upgrade to phased execution

Prerequisites/resources
n/a

SoW (scope of work)

• add aggregationMethod field to the plugin metadata
• make time-sync check for aggregation method in the plugin metadata instead of in the aggregation config block
• test cases added

Acceptance criteria

• test manifests execute correctly using aggregation methods loaded from plugin metadata.

[zanete]

@jmcook1186 @narekhovhannisyan please review the PR that @manushak will link shortly :)

[zanete]

need to update the IF core an instead of Joseph as second reviewer, we can ask @MariamKhalatova to review instead so that this can unblock the manifest updates