resolves #7839

In the CLI integration, MetricFlow will issue dry run queries as
part of its warehouse-level validation of the semantic manifest,
including all semantic model and metric definitions.

In most cases, issuing an `explain` query is adequate, however,
BigQuery does not support the `explain` keyword and so we cannot
simply pre-pend `explain` to our input queries and expect the
correct behavior across all contexts.

This commit adds a dry_run() method to the BaseAdapter which mirrors
the execute() method in that it simply delegates to the ConnectionManager.
It also adds a working implementation to the SQLConnectionManager and
includes a few test cases for adapter maintainers to try out on their own.

The current implementation should work out of the box with most
of our adapters. BigQuery will require us to implement the dry_run
method on the BigQueryConnectionManager, and community-maintained
adapters can opt in by enabling the test and ensuring their own
implementations work as expected.

Note - we decided to make these concrete methods that throw runtime
exceptions for direct descendants of BaseAdapter in order to avoid
forcing community adapter maintainers to implement a method that does
not currently have any use cases in dbt proper.

The common pattern for engine-specific SQL statement construction
in dbt is to provide a default macro which can then be overridden
on a per-adapter basis by either adapter maintainers or end users.
The advantage of this is users can take advantage of alternative
SQL syntax for performance or other reasons, or even to enable
local usage if an engine relies on a non-standard expression and
the adapter maintainer has not updated the package.

Although there are some risks here they are minimal, and the benefit
of added expressiveness and consistency with other similar constructs
is clear, so we adopt this approach here.

…run.

The validate_sql name has less chance of colliding with dbt's
command nomenclature, both now and in some future where we have
dry-run operations.