TimeUnit conversion for BigQuery compatibility

[devlucasc]

These changes allow to set option that make conversion of timestamp in long format using a specified precision, for example, microseconds.
The setting of precision option can make avro compatible with BigQuery format, that reads timestamp in microseconds.

[codecov-io]

Codecov Report

Merging #271 into master will increase coverage by 0.09%.
The diff coverage is 100%.

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  Branches       43      48       +5     
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+ Hits          296     300       +4     
  Misses         25      25

[gengliangwang]

Hi @devlucasc ,
In my opinion:

1. If we should add this option, we need to add it for all the data sources, instead of just for AVRO.
2. We can use .map or SQL statement to change the output timestamp unit, which is simple and straight forward to me.

[devlucasc]

Hi @gengliangwang

1. The problem only occur in avro data sources
2. Map is not good when has a complex structure as arrays, structs or array of structs that contains a timestamp field. In this case is needed to make map, flatmap and explode to treat.

Kind regards.