[SPARK-20198][SQL] Remove the inconsistency in table/function name conventions...

[SPARK-20198][SQL] Remove the inconsistency in table/function name conventions in SparkSession.Catalog APIs

### What changes were proposed in this pull request?
Observed by felixcheung , in `SparkSession`.`Catalog` APIs, we have different conventions/rules for table/function identifiers/names. Most APIs accept the qualified name (i.e., `databaseName`.`tableName` or `databaseName`.`functionName`). However, the following five APIs do not accept it.
- def listColumns(tableName: String): Dataset[Column]
- def getTable(tableName: String): Table
- def getFunction(functionName: String): Function
- def tableExists(tableName: String): Boolean
- def functionExists(functionName: String): Boolean

To make them consistent with the other Catalog APIs, this PR does the changes, updates the function/API comments and adds the `params` to clarify the inputs we allow.

### How was this patch tested?
Added the test cases .

Author: Xiao Li <gatorsmile@gmail.com>

Closes #17518 from gatorsmile/tableIdentifier.