rowexec: implement deep copies for EncDatumRow

[asubiotto]

Throughout the row execution engine, there are interfaces that specify that a row may only be reused until the next call to a specific interface method. For example Next in RowSource or Row in RowIterator. Callers that want to buffer rows call CopyRow with the returned row for reuse.

CopyRow only performs a shallow copy. This is fine most of the time because processors tend to only reuse the EncDatum object, rather than any of its fields. However, #43145 ran into problems when trying to reuse the encoded field since the slice is not copied in CopyRow. This is dangerous because this could result in a row changing under the caller even after a CopyRow. To satisfy the interface contract, we need to implement deep copies for EncDatumRows.

To provide deep copies of EncDatum, we need to also add a Copy method to the Datum interface with implementations for each type.

[asubiotto]

From the Next contract:

<...>
	// EncDatumRows returned by Next() are only valid until the next call to
	// Next(), although the EncDatums inside them stay valid forever.
	//
<...>
	Next() (sqlbase.EncDatumRow, *execinfrapb.ProducerMetadata)

So the current behavior is acceptable.