### What changes were proposed in this pull request?
This PR adds `SparkSession.executeCommand` API for external datasource to execute a random command like
```
val df = spark.executeCommand("xxxCommand", "xxxSource", "xxxOptions")
```
Note that the command doesn't execute in Spark, but inside an external execution engine depending on data source. And it will be eagerly executed after `executeCommand` called and the returned `DataFrame` will contain the output of the command(if any).
### Why are the changes needed?
This can be useful when user wants to execute some commands out of Spark. For example, executing custom DDL/DML command for JDBC, creating index for ElasticSearch, creating cores for Solr and so on(as HyukjinKwon suggested).
Previously, user needs to use an option to achieve the goal, e.g. `spark.read.format("xxxSource").option("command", "xxxCommand").load()`, which is kind of cumbersome. With this change, it can be more convenient for user to achieve the same goal.
### Does this PR introduce any user-facing change?
Yes, new API from `SparkSession` and a new interface `ExternalCommandRunnableProvider`.
### How was this patch tested?
Added a new test suite.
Closes#27199 from Ngone51/dev-executeCommand.
Lead-authored-by: yi.wu <yi.wu@databricks.com>
Co-authored-by: Xiao Li <gatorsmile@gmail.com>
Co-authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>
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