[gcp] Fix BQ BIGNUMERIC type support

[RustedBones]

BIGNUMERIC is currently not properly supported.

• toTable cannot infer a schema of type BIGNUMERIC
• reading from a BIGNUMERIC table returns badly converted BigDecimal (using wrong scale/precision)

Introduce new type BigNumeric to solve this issue

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 61.12%. Comparing base (b52036a) to head (e09ffa0).

Files	Patch %	Lines
.../scala/com/spotify/scio/bigquery/StorageUtil.scala	0.00%	6 Missing :warning:
...cala/com/spotify/scio/bigquery/types/package.scala	73.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5428 +/- ## ========================================== + Coverage 61.09% 61.12% +0.03% ========================================== Files 310 311 +1 Lines 11060 11075 +15 Branches 769 764 -5 ========================================== + Hits 6757 6770 +13 - Misses 4303 4305 +2 ```

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