<!--- This environment context section helps us quickly review your PR. 
      Please take a minute to fill-out this information. -->

## Environment
**Liquibase Version**: 4.3.4

**Liquibase Integration & Version**: CLI

**Database Vendor & Version**: PostgreSQL 12

**Operating System Type & Version**: Ubuntu 20.04

## Description
Hello,

I'm facing the same problem that's described in [this forum thread](https://forum.liquibase.org/t/access-postgis-from-different-schema/4708/3) (I did not reply in the forum because the number of links in a comment is restricted for new users). I'm using postgis as well but I guess it's the same for any extension that you would install on the public schema. The problem is that liquibase sets the search_path to only the default schema.

In the code, this is done in [DatabaseUtils#initializeDatabase](https://github.com/liquibase/liquibase/blob/master/liquibase-core/src/main/java/liquibase/database/core/DatabaseUtils.java#L36), which is called from [PostgresDatabase#rollback](https://github.com/liquibase/liquibase/blob/master/liquibase-core/src/main/java/liquibase/database/core/PostgresDatabase.java#L421) and the rollback function seems to be called for each changeset, after the preconditions are checked, in [ChangeSet#execute](https://github.com/liquibase/liquibase/blob/master/liquibase-core/src/main/java/liquibase/changelog/ChangeSet.java#L614).

I think that the problem was introduced with this commit: https://github.com/liquibase/liquibase/commit/feb2412. Before that, the search_path was not forced for each changeset, and there was no problem using an extension from the public schema without using prefixes.

I guess that there was a good reason to force the search_path, but I think that there should be an option to override the default behavior and to allow the user to set a custom search_path.

## Steps To Reproduce
With postgis:

* Add the postgis extension to the public schema: `CREATE EXTENSION postgis;`
* Run liquibase on another schema (`my_schema`) with the following changeset:

```
create table my_schema.test_table (
     a_box        Box2D
)
```

Liquibase will fail with the following error:

```
ERROR: type "box2d" does not exist
```

Thanks.


if (((defaultCatalogName != null) || (defaultSchemaName != null)) && !(database.getConnection() instanceof
              OfflineConnection)) {
          if (database instanceof OracleDatabase) {
              String schema = defaultCatalogName;
              if (schema == null) {
                  schema = defaultSchemaName;
              }
              Scope.getCurrentScope().getSingleton(ExecutorService.class).getExecutor("jdbc", database).execute(
                  new RawSqlStatement("ALTER SESSION SET CURRENT_SCHEMA=" +
                      database.escapeObjectName(schema, Schema.class)));
          } else if (database instanceof PostgresDatabase && defaultSchemaName != null) {
              Scope.getCurrentScope().getSingleton(ExecutorService.class).getExecutor("jdbc", database).execute(new RawSqlStatement("SET SEARCH_PATH TO " + database.escapeObjectName(defaultSchemaName, Schema.class)));
          } else if (database instanceof AbstractDb2Database) {
              String schema = defaultCatalogName;
              if (schema == null) {
                  schema = defaultSchemaName;
              }
              Scope.getCurrentScope().getSingleton(ExecutorService.class).getExecutor("jdbc", database).execute(new RawSqlStatement("SET CURRENT SCHEMA "
                      + schema));
          } else if (database instanceof MySQLDatabase) {
              String schema = defaultCatalogName;
              if (schema == null) {
                  schema = defaultSchemaName;
              }
              Scope.getCurrentScope().getSingleton(ExecutorService.class).getExecutor("jdbc", database).execute(new RawSqlStatement("USE " + schema));
          }
      }
  }
}