Allow the possibility to optionnaly add "AS" in the From clause for CustomDialect

Some dialect like Oracle do not support "AS" in the from clause and failed for some query :

eg that is working or Oracle :
```sql
SELECT SRC.*, SYSDATE AS CopyDate FROM TPCH_IN.ORDERS_FLAT SRC WHERE 1 = 1 AND O_ORDERKEY BETWEEN 100 and 1000
```
whereas this is not working (on Oracle)
```sql
SELECT SRC.*, SYSDATE AS CopyDate FROM TPCH_IN.ORDERS_FLAT AS SRC WHERE 1 = 1 AND O_ORDERKEY BETWEEN 100 and 1000
```

Currently the sqlparser force the addition of the keyword AS between the table and it's alias even the original query does not contains "AS" in its from clause.

I don't see the ability to create a custom dialect to avoid this behavior.

I am not sure but i think the "problem" could be here :
https://github.com/apache/datafusion-sqlparser-rs/blob/62495f2f0d998690495f111063a0b0a3466ffa69/src/ast/query.rs#L662

Beginne in src/ast/query.rs ungefähr bei Zeile 662 und verfolge, wie Tabellenaliase für CustomDialect gerendert werden. Prüfe die nahegelegenen Dialekt- und Rendering-Tests und verifiziere anschließend, dass ein benutzerdefinierter Dialekt AS in FROM-Aliasen auslassen kann, während das Verhalten bestehender Dialekte unverändert bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.