Support copy protocol text format with CopyFrom

Unlike regular queries the copy protocol requires all values be formatted in binary or text. It does not allow some types formatted as text and some as binary.

`CopyFrom()` currently always uses the binary protocol. This requires all types to be registered and to support the binary format.

This means that enum and other string-like types cannot be used without registration even though they work with normal query functions. See #1338.

In addition, those who simply wish to pass strings into copy (apparently how `pq.CopyIn()` worked) without any type conversion are unable to do so. See #1346.

If `CopyFrom()` supported the text protocol both these issues would be resolved.

`CopyFrom()` could automatically fallback the text protocol as needed. This would be most convenient for the user and would entail no interface changes.

Another option is to allow the caller to determine the desired format. This would allow the caller to ensure the binary format is being used (i.e. ensure the fastest mode is being used) or get the feedback of an error message.

I lean toward the automatic fallback option.

Start at the CopyFrom entry point and trace how the current binary copy protocol is selected and how type registration affects it. Read the context from issues #1338 and #1346, then compare the automatic-fallback and caller-selected-format proposals; the work is done when the chosen behavior supports text-format values without requiring registration and has corresponding coverage.

Written by the indexing model from the issue text.