[Update] Support update syntax

[EmmyMiao87]

Proposed changes

[Update] Support update syntax

The current update syntax only supports updating the filtered data of a single table.

Syntax:

 * UPDATE table_reference
 *     SET assignment_list
 *     [WHERE where_condition]
 *
 * value:
 *     {expr}
 *
 * assignment:
 *     col_name = value
 *
 * assignment_list:
 *     assignment [, assignment] ...

Example
Update unique_table
     set v1=1
     where k1=1

New Frontend Config: enable_concurrent_update
This configuration is used to control whether multi update stmt can be executed concurrently in one table.
Default value is false which means A table can only have one update task being executed at the same time.
If users want to update the same table concurrently,
  they need to modify the configuration value to true and restart the master frontend.
Concurrent updates may cause write conflicts, the result is uncertain, please be careful.

The main realization principle:
1. Read the rows that meet the conditions according to the conditions set by where clause.
2. Modify the result of the row according to the set clause.
3. Write the modified row back to the table.

Some restrictions on the use of update syntax.
1. Only the unique table can be updated
2. Only the value column of the unique table can be updated
3. The where clause currently only supports single tables

Possible risks:
1. Since the current implementation method is a row update,
     when the same table is updated concurrently, there may be concurrency conflicts which may cause the incorrect result.
2. Once the conditions of the where clause are unsatisfactory, it is likely to cause a full table scan and affect query performance.
   Please pay attention to whether the column in the where clause can match the index when using it.

[Docs][Update] Add update document and sql-reference

Fixed #6229

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Checklist

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• I have created an issue on (Fix Support update in unique data model #6229) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• If these changes need document changes, I have updated the document
• Any dependent changes have been merged

[morningman]

You change it to return null, but don' t change the caller to handle null? Such as in StreamLoadPlanner?

[EmmyMiao87]

I unified handle this in the OlapTableSink.

[morningman]

You forgot to add KW_UPDATE

[morningman]

LGTM

[github-actions]

PR approved by anyone and no changes requested.