[SPARK-12476][SQL] Implement JdbcRelation#unhandledFilters for removing unnecessary Spark Filter

Input: SELECT * FROM jdbcTable WHERE col0 = 'xxx'

Current plan:
```
== Optimized Logical Plan ==
Project [col0#0,col1#1]
+- Filter (col0#0 = xxx)
   +- Relation[col0#0,col1#1] JDBCRelation(jdbc:postgresql:postgres,testRel,[Lorg.apache.spark.Partition;2ac7c683,{user=maropu, password=, driver=org.postgresql.Driver})

== Physical Plan ==
+- Filter (col0#0 = xxx)
   +- Scan JDBCRelation(jdbc:postgresql:postgres,testRel,[Lorg.apache.spark.Partition;2ac7c683,{user=maropu, password=, driver=org.postgresql.Driver})[col0#0,col1#1] PushedFilters: [EqualTo(col0,xxx)]
```

This patch enables a plan below;
```
== Optimized Logical Plan ==
Project [col0#0,col1#1]
+- Filter (col0#0 = xxx)
   +- Relation[col0#0,col1#1] JDBCRelation(jdbc:postgresql:postgres,testRel,[Lorg.apache.spark.Partition;2ac7c683,{user=maropu, password=, driver=org.postgresql.Driver})

== Physical Plan ==
Scan JDBCRelation(jdbc:postgresql:postgres,testRel,[Lorg.apache.spark.Partition;2ac7c683,{user=maropu, password=, driver=org.postgresql.Driver})[col0#0,col1#1] PushedFilters: [EqualTo(col0,xxx)]
```

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #10427 from maropu/RemoveFilterInJdbcScan.