chore: move row value classes out of row_data

[igorbernstein2]

This is in preparation for extracting row merging into a separate class. See #628

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• google/cloud/bigtable/row.py:1072, tag bigtable_api_row_data_cells
• google/cloud/bigtable/row.py:1099, tag bigtable_api_row_find_cells
• google/cloud/bigtable/row.py:1137, tag bigtable_api_row_cell_value
• google/cloud/bigtable/row.py:1179, tag bigtable_api_row_cell_values

You are about to delete 4 region tags.

• google/cloud/bigtable/row_data.py:184, tag bigtable_api_row_data_cells
• google/cloud/bigtable/row_data.py:211, tag bigtable_api_row_find_cells
• google/cloud/bigtable/row_data.py:249, tag bigtable_api_row_cell_value
• google/cloud/bigtable/row_data.py:291, tag bigtable_api_row_cell_values

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