A core component of `sqldb/v2` useage, is that the package allows and
requires that the callsite defines a `sqldb.MigrationStream` that will
be run during the initialization of the `sqldb/v2 database instance.
The `sqldb.MigrationStream` defines the exact sql migrations to run,
as well as additional code migrations will be run after each individual
migration version.

This commit introduces the core definition of the migration stream for
`litd`, and will be further exteded in upcoming commits that will
introduce kvdb to sql code migration.

Note that as of this commit, as no part of the `litd` codebase uses the
`sqldb/v2` database instances, this migration stream is not yet
used.

The `sqldb/v2` package now provides a definition for the `BackendType`
type. As useage of `sqldb/v2` requires useage of that type, we update
`litd` to use the `BackendType` definition from `sqldb/v2`, instead of
it's own definition.

The upcoming implementation of `sqldb/v2` will extensively create
a `Queries` object on the fly. To make more intuitive how to create
the queries object for specific database types, we introduce a
`NewForType` helper method.
This also mimics how `tapd` creates the `Queries` object, and in order
not let `litd` have it's own definition of how `Queries` object are
created on the fly, the upcoming `sqldb/v2` usage will utilize this
helper method.

As we will change the `accounts`, `session` & `firewalldb` packages to
use the `sqldb/v2` package, we need to make those packages use the
`sqldb/v2` `PostgresStore` when setting up their test postgres
databases, instead of `litd`'s own `PostgresStore` version.

In order to enable that functionality, we add a new helper function that
creates a `PostgresStore` using the `sqldb/v2` package, in addition to
helper function that creates a `PostgresStore` using the `litd`
version.

Once we have shifted all of `litd`'s code to use the `sqldb/v2`
definition, we will remove the `litd` version.

Update the accounts package to use the `sqldb/v2` package instead of the
older version.

Update the session package to use the `sqldb/v2` package instead of the
older version.

Previous commits had forgotten to add the `ListAllKVStoresRecords` query
to the `firewalldb.SQLKVStoreQueries` interface. As that is required to
make the query useable when defining the `sqldb/v2`
`TransactionExecutor` for the `firewalldb` package, this commit
adds it to the interface.

rename `sqlStore` to `store` in the firewalldb sql migration test file,
to make the name shorted. This is done in preparation for future commits
which will lengthen the lines where `sqlStore` is used, which otherwise
would make the lines exceed the 80 character limit.

Update the firewalldb package to use the `sqldb/v2` package instead of
the older version.

As we've now switched over to using sqldb v2 for most of the db objects,
we can remove a lot of deprecated code that's no longer used in the litd
project. This commit removes that code.

As the legacy `NewTestPostgresDB` function is no longer used and has
been removed, it no longer makes sense to have a `V2` suffix on the
`NewTestPostgresV2DB` function. This commit renames it to
`NewTestPostgresDB`, to indicate that this function now replaces the
legacy function.