feat: add more validation for filtering

[scotwells]

Filtering with an unknown field on an object would lead to a "503 Service Unavailable" error instead of an "Invalid request" error.

This change adjusts the filter validation to confirm that all fields under an object are actually filterable fields so appropriate validation is returned.

Current Behavior:

$ datumctl activity query --project datum-cloud --filter 'objectRef.resources == "test"' -v 6
...
I0120 12:36:20.074227 54523 round_trippers.go:632] "Response" verb="POST" url="https://api.staging.env.datum.net/apis/resourcemanager.miloapis.com/v1alpha1/projects/datum-cloud/control-plane/apis/activity.miloapis.com/v1alpha1/auditlogqueries" status="503 Service Unavailable" milliseconds=427
error: query failed: Failed to execute query. Please try again later or contact support for help.

New Behavior:

$ datumctl activity query --project datum-cloud --filter 'objectRef.resources == "test"'
error: query failed: AuditLogQuery.activity.miloapis.com "" is invalid: spec.filter: Invalid value: "objectRef.resources == "test"": Invalid filter: field 'objectRef.resources' is not available for filtering. Available fields for objectRef: [objectRef.resource objectRef.name objectRef.apiGroup objectRef.namespace]. Available fields: auditID, verb, requestReceivedTimestamp, objectRef.namespace, objectRef.resource, objectRef.name, user.username, user.groups, responseStatus.code. See https://cel.dev for CEL syntax

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Relates to datum-cloud/enhancements#536