refactor: one place for Key Vault collections and paging in the providers

- ObjectType.Collection names a type's segment in Key Vault URLs and links (secrets, keys, certificates), instead of four copies in ref, azure and fake.
- Azure lists through one pager helper instead of eight hand-written page loops; Value is VersionedValue without the version, rather than the two calling each other.
- Databricks' Key Vault fallback check lives in fallbackVault.
- The repo provider reads .lazysecrets.yaml in one place, builds plain and sops objects in one loop, and parses a file's sops metadata from the bytes Objects already read.

internal/provider/provider.go

@@ -84,6 +84,20 @@ const (
 	TypeCertificate ObjectType = "Certificate"
 )
 
+// Collection is the type's path segment in Key Vault URLs and links, e.g.
+// "secrets".
+func (t ObjectType) Collection() string {
+	switch t {
+	case TypeSecret:
+		return "secrets"
+	case TypeKey:
+		return "keys"
+	case TypeCertificate:
+		return "certificates"
+	}
+	return ""
+}
+
 // Container holds objects: a Key Vault or a Databricks secret scope.
 type Container struct {
 	Source Source