Added ListGridpoolEnergySchedules and ListMicrogridSensors RPCs (#99)

## Summary

This PR extends the Platform Assets API with market-topology and
Gridpool-related metadata needed by trading, forecasting, reporting, and
operational tooling. The PR also adds Gridpool energy schedules and
microgrid sensor listing support.

## Changes

- Added `ListMarketTopologyRelations` for querying market-topology
relations within the current enterprise scope.
- Added `MarketTopologyRelation` as the canonical relation model between
Gridpools, Microgrids, and Market Locations.
- Added `MarketParticipation` and `MarketParticipationType` to describe
use-case-specific participation of a market-topology relation.
- Added `ListGridpoolEnergySchedules` for retrieving static Gridpool
energy schedules.
- Added `GridpoolEnergySchedule`, including counterparty balancing
group, Gridpool and counterparty delivery areas, direction, validity
period, cancellation timestamp, delivery duration, and scheduled
active-power time series.
- Added `GridpoolEnergyScheduleDirection` to describe import/export
direction from the perspective of the Frequenz balancing group.
- Added `ListMicrogridSensors` for listing sensors belonging to a
specific microgrid.
- Updated service-level documentation to describe both platform asset
metadata and market-topology metadata.
- Aligned request filters with the existing nested filter-message
pattern used by the API.

## Notes

`MarketTopologyRelation` intentionally supports relations without a
Gridpool. These represent Microgrid-to-Market-Location links used for
deployment, metering validation, reporting comparison, or local
optimization, and do not imply market participation.

If `gridpool_id` is set on a `MarketTopologyRelation`, the relation
represents participation in a Gridpool context. In that case,
`delivery_area` is required and `participations` may describe
use-case-specific validity.

Gridpool energy schedules use a schedule-level `delivery_duration`. Each
time-series entry contains only a start timestamp and applies until
`start_time + delivery_duration`.

The API does not expose derived active/inactive schedule status. Clients
can derive effective status from `validity_period`, `cancel_time`, and
their own evaluation time.

Author: thomas-nicolai-frequenz

RELEASE_NOTES.md

@@ -2,15 +2,14 @@
 
 ## Summary
 
-This release introduces Gridpool assignment discovery to the Platform Assets API.
+This release renames `MarketLocationSelector` to `MarketLocation`
 
 ## Upgrading
 
-- Minor documentation changes.
+- Rename `MarketLocationSelector` to `MarketLocation`.
 
 ## New Features
 
-- New ListGridpoolAssignments RPC has been added
 
 ## Bug Fixes