Cockroachdb copy_records_to_table support?

* **asyncpg version**: v0.21.0
* **PostgreSQL version**: CCL v20.1.8 @ 2020/10/21 15:46:38 (go1.13.9)
* **Python version**: 3.8
* **Platform**: macos
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes

----

EDIT:
**Confirmed cockroachdb does not support the COPY ... FROM STDIN syntax.** (https://github.com/cockroachdb/cockroach/issues/16392)

Reasoning is:
_"because it's 1) complicated and 2) slow. All the data goes through a single connection. CockroachDB provides an IMPORT PGDUMP statement which is much faster and can operate in parallel."_

Hopefully one of the maintainers can comment on this.

----

Hello, I'm not sure if this has been asked before, but I'm having issues running copy_records_to_table into a cockroachdb table/temp table. Here's a code snippet which will reproduce the issue when running a local cockroachdb.

Exception:
```
PostgresSyntaxError at or near "(": syntax error
DETAIL: source SQL:
COPY "cool_table" FROM STDIN (FORMAT binary)
^
```

```python
import asyncio
import asyncpg

async def bwrite():
dsn = "postgres://root@localhost:26257/postgres"
conn: asyncpg.Connection = await asyncpg.connect(dsn)
await conn.execute("SET experimental_enable_temp_tables = on")
await conn.execute(
"""CREATE TEMP TABLE cool_table(
objectid int8,
CONSTRAINT "primary" PRIMARY KEY (objectid ASC)
);"""
)
try:
this_res = await conn.fetch("SHOW TABLES from pg_temp")
print(this_res)
await conn.copy_records_to_table(
"cool_table", records=[(12345,), (67890,)]
)
except Exception as e:
print(e)

if __name__ == "__main__":
# build: CCL v20.1.8 @ 2020/10/21 15:46:38 (go1.13.9)
loop = asyncio.get_event_loop()
res = loop.run_until_complete(bwrite())

```

Start with asyncpg.Connection.copy_records_to_table and the generated COPY "cool_table" FROM STDIN (FORMAT binary) statement shown in the issue. Use the provided reproduction with CockroachDB and a temporary table as the acceptance case; done means the requested support works or the incompatibility and supported alternative are clearly established.

Written by the indexing model from the issue text.