Set postgresql ALPN on SSL contexts created by asyncpg

[DurandA]

PostgreSQL 17+ rejects direct SSL connections that don't negotiate the postgresql ALPN protocol:

LOG:  received direct SSL connection request without ALPN protocol negotiation extension

asyncpg never set ALPN, so direct_tls=True (or sslnegotiation=direct) only worked through TLS-terminating proxies, not against the server itself. The existing direct TLS tests all go through the test proxy, which is why this wasn't caught.

This sets ALPN on the contexts asyncpg creates (string ssl modes and ssl=True), like libpq does. It's sent for both negotiation modes, same as libpq; servers without ALPN support just ignore it.

User-provided SSLContexts are left alone, since asyncpg doesn't mutate them elsewhere either. The direct_tls docs now say to call ctx.set_alpn_protocols(['postgresql']) on them.

Added test_direct_tls_native, which connects straight to the test cluster with direct_tls=True (skipped before PG 17). Ran the SSL connection tests against PostgreSQL 18: they pass with the change, and the new test fails for every mode without it.

Fixes #1371

[elprans]

Please guard ALPN setup on builds without ALPN; default TCP connections otherwise fail.

[elprans]

set_alpn_protocols() raises NotImplementedError without ALPN, breaking default ssl='prefer' connections. Guard both calls:

Suggested change

	ssl.set_alpn_protocols(_ALPN_PROTOCOLS)
	elif ssl is True:
	ssl = ssl_module.create_default_context()
	ssl.set_alpn_protocols(_ALPN_PROTOCOLS)
	if ssl_module.HAS_ALPN:
	ssl.set_alpn_protocols(_ALPN_PROTOCOLS)
	elif ssl is True:
	ssl = ssl_module.create_default_context()
	if ssl_module.HAS_ALPN:
	ssl.set_alpn_protocols(_ALPN_PROTOCOLS)

[DurandA]

Good catch, thanks. Guarded both calls in fa80694. I checked it by running the SSL tests with HAS_ALPN patched to False and set_alpn_protocols raising NotImplementedError: they pass with the guard and fail without it.