<!--- 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.


private String dbFullVersion=null;
    public static final String PRODUCT_NAME = "PostgreSQL";
    public static final int MINIMUM_DBMS_MAJOR_VERSION = 9;
    public static final int MINIMUM_DBMS_MINOR_VERSION = 2;
    /**
     * Maximum length of PostgresSQL identifier.
     * For details see https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS.
     */
    static final int PGSQL_PK_BYTES_LIMIT = 63;
    static final String PGSQL_PK_SUFFIX = "_pkey";
    static final Charset CHARSET = Scope.getCurrentScope().getFileEncoding();
    private static final int PGSQL_DEFAULT_TCP_PORT_NUMBER = 5432;
    private static final Logger LOG = Scope.getCurrentScope().getLog(PostgresDatabase.class);
    /**
     * Represents Postgres DB types.
     * Note: As we know COMMUNITY, RDS and AWS AURORA have the same Postgres engine. We use just COMMUNITY for those 3
     *       If we need we can extend this ENUM in future
     */
    public enum DbTypes {
        EDB, COMMUNITY, RDS, AURORA
    }
    private Set<String> systemTablesAndViews = new HashSet<>();
    private Set<String> reservedWords = new HashSet<>();