[SPARK-51743] Add describe_(database|table), show_(database|table), explain sql test and answer files

### What changes were proposed in this pull request?

This PR aims to add the following test coverage.

```sql
DESCRIBE DATABASE
DESCRIBE TABLE
EXPLAIN
SHOW DATABASE
SHOW TABLE
```

### Why are the changes needed?

To improve the test coverage.

### Does this PR introduce _any_ user-facing change?

No, this is a test-only addition.

### How was this patch tested?

Pass the CIs and do manual check.
```
$ swift test --filter SQLTests
...
􀟈  Suite SQLTests started.
􀟈  Test runAll() started.
􀟈  Test testRemoveID() started.
􁁛  Test testRemoveID() passed after 0.006 seconds.
binary.sql
cache.sql
clear_cache.sql
date.sql
describe_database.sql
describe_function.sql
describe_query.sql
describe_table.sql
explain.sql
floating.sql
integral.sql
pipesyntax.sql
select.sql
show_databases.sql
show_tables.sql
string.sql
uncache.sql
􁁛  Test runAll() passed after 0.529 seconds.
􁁛  Suite SQLTests passed after 0.529 seconds.
􁁛  Test run with 2 tests passed after 0.529 seconds.
```

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #46 from dongjoon-hyun/SPARK-51743.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Tests/SparkConnectTests/Resources/queries/describe_database.sql

@@ -0,0 +1 @@
+DESCRIBE DATABASE default

Tests/SparkConnectTests/Resources/queries/describe_database.sql.json

@@ -0,0 +1 @@
+[["Catalog Name","spark_catalog"],["Namespace Name","default"],["Comment","default database"],["Location","file:\/opt\/spark\/work-dir\/spark-warehouse"],["Owner","185"]]

Tests/SparkConnectTests/Resources/queries/describe_table.sql

@@ -0,0 +1 @@
+DESCRIBE TABLE testcache

Tests/SparkConnectTests/Resources/queries/describe_table.sql.json

@@ -0,0 +1 @@
+[["col","int",null]]

Tests/SparkConnectTests/Resources/queries/explain.sql

@@ -0,0 +1 @@
+EXPLAIN EXTENDED select k, sum(v) from values (1, 2), (1, 3) t(k, v) group by k

Tests/SparkConnectTests/Resources/queries/explain.sql.json

@@ -0,0 +1 @@
+[["== Parsed Logical Plan ==\n'Aggregate ['k], ['k, unresolvedalias('sum('v))]\n+- SubqueryAlias t\n   +- LocalRelation [k#, v#]\n\n== Analyzed Logical Plan ==\nk: int, sum(v): bigint\nAggregate [k#], [k#, sum(v#) AS sum(v)#]\n+- SubqueryAlias t\n   +- LocalRelation [k#, v#]\n\n== Optimized Logical Plan ==\nAggregate [k#], [k#, sum(v#) AS sum(v)#]\n+- LocalRelation [k#, v#]\n\n== Physical Plan ==\nAdaptiveSparkPlan isFinalPlan=false\n+- HashAggregate(keys=[k#], functions=[sum(v#)], output=[k#, sum(v)#])\n   +- Exchange hashpartitioning(k#, 200), ENSURE_REQUIREMENTS, [plan_id=]\n      +- HashAggregate(keys=[k#], functions=[partial_sum(v#)], output=[k#, sum#])\n         +- LocalTableScan [k#, v#]\n"]]

Tests/SparkConnectTests/Resources/queries/show_databases.sql

@@ -0,0 +1 @@
+SHOW DATABASES

Tests/SparkConnectTests/Resources/queries/show_databases.sql.json

@@ -0,0 +1 @@
+[["default"]]

Tests/SparkConnectTests/Resources/queries/show_tables.sql

@@ -0,0 +1 @@
+SHOW TABLES

Tests/SparkConnectTests/Resources/queries/show_tables.sql.json

@@ -0,0 +1 @@
+[["","testcache","true"]]