Move smart agent config validation from unmarshal

[dmitryax]

This fixes receiver_creator failing to start smartagent receivers since 0.124.0 with the following error:

2025-05-01T09:35:43.989Z	info	receivercreator@v0.124.1-0.20250415150511-f840bdff19ca/observerhandler.go:201	starting receiver	{"name": "smartagent/coredns", "endpoint": "10.42.0.8", "endpoint_id": "k8s_observer/c7a1646a-969d-4e5d-9bbd-a95574b903b3", "config": {"extraDimensions":{"metric_source":"k8s-coredns"},"port":9153,"type":"coredns"}}
2025-05-01T09:35:43.991Z	error	receivercreator@v0.124.1-0.20250415150511-f840bdff19ca/observerhandler.go:217	failed to start receiver	{"receiver": "smartagent/coredns", "error": "failed creating endpoint-derived receiver: Validation error in field 'Config.host': host is a required field (got '')"}
github.com/open-telemetry/opentelemetry-collector-contrib/receiver/receivercreator.(*observerHandler).startReceiver
	github.com/open-telemetry/opentelemetry-collector-contrib/receiver/receivercreator@v0.124.1-0.20250415150511-f840bdff19ca/observerhandler.go:217
github.com/open-telemetry/opentelemetry-collector-contrib/receiver/receivercreator.(*observerHandler).OnAdd
	github.com/open-telemetry/opentelemetry-collector-contrib/receiver/receivercreator@v0.124.1-0.20250415150511-f840bdff19ca/observerhandler.go:105
github.com/open-telemetry/opentelemetry-collector-contrib/extension/observer/endpointswatcher.(*EndpointsWatcher).updateAndNotifyOfEndpoints
	github.com/open-telemetry/opentelemetry-collector-contrib/extension/observer@v0.124.1/endpointswatcher/endpointswatcher.go:114

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 34.16%. Comparing base (454dff5) to head (e596c6c).

Files with missing lines	Patch %	Lines
pkg/receiver/smartagentreceiver/config.go	87.50%	1 Missing and 1 partial ⚠️

❌ Your project check has failed because the head coverage (34.16%) is below the target coverage (40.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (454dff5) and HEAD (e596c6c). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (454dff5)	HEAD (e596c6c)
	35	30

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6187       +/-   ##
===========================================
- Coverage   44.52%   34.16%   -10.37%     
===========================================
  Files         390      390               
  Lines       27014    26993       -21     
===========================================
- Hits        12028     9221     -2807     
- Misses      14148    17152     +3004     
+ Partials      838      620      -218     

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