Add DefaultPostgreSQLProviderFeatures with the 65535 placeholder limit

[Chengxuan]

PostgreSQL caps a single statement at 65535 bind parameters. The limit comes from the wire protocol. InsertMany already chunks multi-row inserts when SQLFeatures.MaxPlaceholders is set, but every PostgreSQL provider has to remember to set it.

This adds, next to BuildPostgreSQLOptimizedUpsert:

• PostgreSQLMaxPlaceholders = 65535
• DefaultPostgreSQLProviderFeatures(), returning DefaultSQLProviderFeatures() with $N placeholders, multi-row insert, MaxPlaceholders and the PostgreSQL optimized upsert builder set

Providers can start from it and add their own features on top (lock SQL, ILIKE, sequence column filtering), for example:

func (psql *Postgres) Features() dbsql.SQLFeatures {
	features := dbsql.DefaultPostgreSQLProviderFeatures()
	features.AcquireLock = func(lockName string) string { ... }
	return features
}

No behavior changes for existing callers.

[onelapahead]

This makes sense to me, but might defer to @awrichar on the specific features being proposed as defaults