Add LOAD_FILE() function

[morgo]

Feature Request

Is your feature request related to a problem? Please describe:

Not a high priority, but apart from SOUNDEX() the LOAD_FILE function is the only string function not supported by TiDB.

In MySQL it is rarely used because of security implications.

Describe the feature you'd like:

LOAD_FILE as in MySQL 5.7: https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_load-file

Describe alternatives you've considered:

The alternative is to document as won't support.

Teachability, Documentation, Adoption, Migration Strategy:

[breezewish]

LOAD_FILE is used in RQG to generate blob payload.