fix: require type for "string" fields; don't for "any" fields

[signekb]

• fixes tableSchemaFieldString should require type; tableSchemaFieldAny should not #1147

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This adds type as required for string fields (aligned with the other typed fields), and removes type as required for any fields. This aligns with the textual description of field types in the spec:

the absence of a type property indicates that the field is of the type “any”