HEMS: add custom implementation

[andig]

fixes #32182

Adds a custom HEMS that sources its limits from plugins instead of fixed config values, so an external logic (http, mqtt, modbus, gpio, …) can supply them at runtime:

hems:
  type: custom
  maxconsumptionpower: # total load limit (W), 0 = no limit
    source: http
    uri: http://steuerbox.local/api/limit
    jq: .maxPower
  curtailedpercent: # allowed feed-in (0..100), 100 = uncurtailed
    source: mqtt
    topic: hems/curtail/percent
  productionnominalmax: 10000 # nominal production power (W), required with curtailedpercent
  interval: 10s

Both plugins are optional, at least one is required. Values are polled every interval and mapped onto api.HEMS the same way fnn does it — curtailedpercent drives both CurtailedPercent() and MaxProductionPower() so the two cannot contradict each other. Dim and curtail sessions are recorded via smartgrid.UpdateSession like the existing integrations.

Note on the original request: getmaxcurrent has no equivalent in api.HEMS, which is watt-based only, so only power limits are covered.

Failsafe behaviour is deliberately left out pending the semantics discussion — a failing plugin read is logged and the previous limit is retained. Out-of-range values (negative power, percent outside 0..100) are rejected the same way.

🤖 Generated with Claude Code

[andig]

@naltatis need to decide how to present the custom choices and which one should be default.

[github-actions]

✅ Build finished.

• Binaries: download artifacts
• Docker: docker pull evcc/evcc:pr-32187

[SolarPower2024]

thanks for helping it build with Github Bot.

tested and confirm that it´s working.