[FEATURE] Join query doesn't support index alias

Is your feature request related to a problem?

When using index alias instead of index name in JOIN query, NPE is thrown from SQL plugin.

What solution would you like?

JOIN query (or any SQL query) can handle index alias without issue.

Do you have any additional context?

Error stacktrace:

[2024-03-06T22:33:27,579][ERROR][o.o.s.l.p.RestSqlAction  ]  Client side error during query execution
java.lang.NullPointerException: Cannot invoke "org.opensearch.sql.legacy.esdomain.mapping.FieldMappings.has(String)" because "fieldMappings" is null
    at org.opensearch.sql.legacy.rewriter.matchtoterm.TermFieldRewriter.visit(TermFieldRewriter.java:129)
    at com.alibaba.druid.sql.ast.expr.SQLIdentifierExpr.accept0(SQLIdentifierExpr.java:65)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.ast.expr.SQLBinaryOpExpr.accept0(SQLBinaryOpExpr.java:99)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock.accept0(MySqlSelectQueryBlock.java:258)
    at com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock.accept0(MySqlSelectQueryBlock.java:246)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.ast.statement.SQLSelect.accept0(SQLSelect.java:85)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.acceptChild(SQLObjectImpl.java:62)
    at com.alibaba.druid.sql.ast.expr.SQLQueryExpr.accept0(SQLQueryExpr.java:55)
    at com.alibaba.druid.sql.ast.SQLObjectImpl.accept(SQLObjectImpl.java:40)
    at org.opensearch.sql.legacy.query.OpenSearchActionFactory.create(OpenSearchActionFactory.java:110)
    at org.opensearch.sql.legacy.plugin.SearchDao.explain(SearchDao.java:52)
    at org.opensearch.sql.legacy.plugin.RestSqlAction.explainRequest(RestSqlAction.java:208)
    at org.opensearch.sql.legacy.plugin.RestSqlAction.lambda$prepareRequest$1(RestSqlAction.java:160)
    at org.opensearch.sql.legacy.plugin.RestSQLQueryAction$1.onFailure(RestSQLQueryAction.java:130)
    at org.opensearch.sql.sql.SQLService.explain(SQLService.java:60)
    at org.opensearch.sql.legacy.plugin.RestSQLQueryAction.lambda$prepareRequest$2(RestSQLQueryAction.java:98)
    ...

Start in org.opensearch.sql.legacy.rewriter.matchtoterm.TermFieldRewriter.visit, where the stack trace reports the null FieldMappings access, and trace how JOIN index names or aliases are resolved. Reproduce the reported JOIN using an index alias, then verify that the query executes without the NullPointerException for aliases as well as index names.