feat: add alert types for config mismatch, erasure set health, KMS, and storage capacity

[Praveenrajmani]

Introduces a new GetAlerts admin API (POST /admin/alerts) that streams alert events from the MinIO server as newline-delimited JSON, using Go 1.23's iter.Seq2 for idiomatic range-over-function iteration.

New types:

• Alert — the top-level alert event with type, timestamp, title, message, dedup key, and deployment/cluster metadata
• AlertDetails — strongly-typed union struct; only one field populated per alert type
• AlertLogOpts — query options (filter by type, check interval, max per node)

Alert types and their detail structs:

• license-expiry (LicenseExpiryDetails) — license ID, expiry time, state
• certificate-expiry (CertificateExpiryDetails) — common name, serial, DNS names, days until expiry
• config-mismatch (ConfigMismatchDetails) — peer address, mismatch type (env/args/endpoints), differing values
• erasure-set-health (ErasureSetHealthDetails) — pool/set ID, online/total drives, read+write quorum, condition
• kms-unavailable (KMSUnavailableDetails) — KMS type
• storage-capacity (StorageCapacityDetails) — free/total bytes, free percent

Also adds AlertInternalSubSys, AlertWebhookSubSys, and AlertKafkaSubSys subsystem constants to parse-config.go for alert routing configuration.

[klauspost]

Some clarification needed.

[shtripat]

looks good overall

[ramondeklein]

@Praveenrajmani Can you please update the description why this is needed? I guess it's being used in AIStor code to send out alerts?

[klauspost]

Honestly it seems overengineered that each alert type must have its own data structure.

// Alert represents a single alert event in the system.
// It captures alert information with contextual metadata including deployment and cluster information.
type Alert struct {
	Type         AlertType    `json:"type"`
	Timestamp    time.Time    `json:"timestamp"`
	Title        string       `json:"title"`
	Message      string       `json:"message"`
	Custom      map[string]string `json:"custom"`
	DeploymentID string       `json:"deploymentId"`
	ClusterName  string       `json:"clusterName"`
	DedupKey     string       `json:"dedupKey,omitempty"`
}

I know you can't extract specific values that easily, but honestly I don't see a need in 99% of cases.

[Praveenrajmani]

Yeah, agree @klauspost we dont want individual types. Have simplified it now. PTAL.