Additional options support for SELECT INTO and LOAD DATA

[GuptaManan100]

1. Support for using SELECT INTO and LOAD DATA on unsharded keyspaces, in addition to already supported bypass queries.

2. Provide additional options support for SELECT INTO and LOAD statements.

From the MySQL docs:

    INTO OUTFILE 'file_name'
        [CHARACTER SET charset_name]
           [{FIELDS | COLUMNS}
                [TERMINATED BY 'string']
                [[OPTIONALLY] ENCLOSED BY 'char']
                [ESCAPED BY 'char']
            ]
            [LINES
                [STARTING BY 'string']
                [TERMINATED BY 'string']
          ]  

This PR adds support for all the optional options following INTO OUTFILE 'file_name'

[shlomi-noach]

@GuptaManan100 can you kindly elaborate a bit on what this PR does?

Provided additional options support for SELECT INTO and LOAD statements.

Which options are these?

Also, support for unsharded keyspaces is added apart from bypass queries.

Not sure I parse the sentence. What are the bypass queries?

[shlomi-noach]

Which options are these?

Ignore that question, I can see the options in sql.y.

https://github.com/vitessio/vitess/pull/6872/files#diff-bfff6f6dd088d8318ba5604a902fdf50ed5d0fe1706b82c166c99a67d0d81d39

[GuptaManan100]

bypass queries are the ones where the user first specifies the shard that they want the query to run on i.e. "use ks@replica:-80".
Before we do any planning we first check if the destination has already been set, if that is the case then we send the query directly. checkout this function https://github.com/vitessio/vitess/blob/master/go/vt/vtgate/planbuilder/bypass.go#L27 and the place where it is called from https://github.com/vitessio/vitess/blob/master/go/vt/vtgate/planbuilder/builder.go#L300

[harshit-gangal]

This would need few end to end test.

[shlomi-noach]

Overall this looks very good to me, though this isn't in my comfort zone. I'd change the wording on error messages, or invest in documentation. What "bypass" is, our users cannot know at this time.

[GuptaManan100]

I have changed the error messages as discussed. I have also added the documentation for "bypass" in my todo list and will do that as a separate PR.