Add SHOW DATABASES command for consistency with SQL standards

Is your feature request related to a problem?

Description:
QuestDB currently supports SHOW TABLES but lacks SHOW DATABASES. This creates inconsistency with standard SQL implementations and can confuse developers coming from other database systems.

Problem:
When working with QuestDB, users have no SQL command to list available databases or confirm which database context they're in. This is particularly confusing for:

• New users exploring the system
• Developers familiar with PostgreSQL/MySQL where SHOW DATABASES is standard
• Tools/clients that expect this command for database discovery

Expected Behavior:

SHOW DATABASES;
-- Should return a list of available databases

Use Case:

Helps users navigating multi-database setups
Standard SQL command expected by developers migrating from PostgreSQL/MySQL
Improves PostgreSQL wire protocol compatibility
Better integration with database management tools
---

### Describe the solution you'd like.

Implement a `SHOW DATABASES` command that returns a result set similar to `SHOW TABLES`.

**Proposed Implementation:**
```sql
SHOW DATABASES;

### Describe alternatives you've considered.


```markdown
**Current Workarounds:**

1. **Check configuration files** - Users must manually inspect `server.conf` or documentation
   - Issue: Not accessible via SQL, requires file system access

2. **Assume default database** - Users work with the default `questdb` database without confirmation
   - Issue: Error-prone, no validation

3. **Use PostgreSQL metadata queries** - Try `SELECT * FROM pg_database`
   - Issue: May not be fully implemented in QuestDB

**Why `SHOW DATABASES` is better:**
- Native SQL command
- Consistent with existing `SHOW TABLES` pattern
- Discoverable and self-documenting
- Works in any SQL client

### Full Name:

Sandeep Kumar

### Affiliation:

Independent Developer

### Additional context

**Similar implementations:**

MySQL:
```sql
SHOW DATABASES;

Start by tracing the existing SHOW TABLES command and the metadata exposed for databases, then review how server.conf and pg_database relate to the requested behavior. Define the result set and database-context semantics, implement SHOW DATABASES, and verify it through a SQL client against single- and multi-database setups.