Add timestamp-nanos avro logical type support in bigquery avro utils.

[claudevdm]

timestamp-nanos avro logical type was introduced in avro 1.12, but beam cannot upgrade avro versions until java 8 support is dropped #34993

There are workarounds here to handle the timestamp-nanos logical type until we can update avro version.

AvroUtils changes:

• toBeamFieldType(): Recognizes timestamp-nanos property and maps to Timestamp.NANOS
• toAvroSchema(): Converts Timestamp.NANOS to Avro schema with timestamp-nanos logical type
• toGenericRecord(): Converts Instant values to nanoseconds since epoch
• toBeamRowStrict(): Converts nanosecond values back to Instant objects

BigQueryAvroUtils changes:

• Refactored timestamp formatting into TimestampPrecision enum
• Added NANOSECONDS precision level alongside existing MILLISECONDS and MICROSECONDS
• Updated convertRequiredField() to handle timestamp-nanos logical type
• Updated formatDatetime() to dynamically format fractional seconds (3, 6, or 9 digits)
• Updated fromGenericAvroSchema() to map timestamp-nanos to BigQuery TIMESTAMP type

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