Find a Tender API client updates

- Convert OCDS releases to types.Opportunity (internal/types); FetchOnce
  and FetchAll now return opportunities, built from each ocid's latest
  release
- Include each item's main classification as well as its additional ones
- Return ErrUnexpectedStatus for non-200 responses and
  ErrRateLimitExceeded for a repeated 429; close discarded bodies
- Stop fetchAll on a repeated next link and close each page's body once
  read rather than deferring in the loop
- Default a zero Since to 24 hours ago
- Merge date repair and releaseIDs into api.go
- Filter --planning/--opportunities/--awards on each procedure's current
  stage, using OCDS release tags; --opportunities previously matched a
  nonexistent "opportunity" tag and returned nothing
- Reject conflicting filters before fetching (cobra flag group)

internal/types/classifications.go

@@ -0,0 +1,8 @@
+package types
+
+type ClassificationCode struct {
+	ID             int64  `db:"id"             json:"id"`
+	IsCPV          bool   `db:"is_cpv"         json:"is_cpv"`
+	Code           string `db:"code"           json:"code"`
+	Classification string `db:"classification" json:"classification"`
+}