parser,ast: Support for authentication plugins

[dveeden]

What problem does this PR solve?

Required changes to support authentication plugins in CREATE USER statements.

What is changed and how it works?

• This puts the authentication plugin in ast.AuthOption.AuthPlugin
• This also fixes support for hex literals for HashString. This improves MySQL compatibility and makes it easier to use with caching_sha2_password which can contain non-ASCII/non-printable characters.

Note that the IDENTIFIED BY PASSWORD <HashString> is still excepted, but when restoring this this gets rewritten as IDENTIFIED BY 'mysql_native_password' AS <HashString>. The former is no longer supported by MySQL 8.0.

Check List

Tests

• Unit test

Code changes

• Has exported variable/fields change

Side effects

• Increased code complexity

Note that this is based on top of:

• Fix running of auth tests #1227
• Caching sha2 #1232

This is needed for pingcap/tidb#24991 to be able to support CREATE USER statements that create users with caching_sha2_password support.

[morgo]

/LGTM

[tiancaiamao]

#1232 is merged, please update master for this one @dveeden

[tiancaiamao]

rest LGTM

[dveeden]

@morgo @tiancaiamao PTAL

[dveeden]

@tiancaiamao could you have another look?

[tiancaiamao]

/LGTM