Add support for SHOW DATABASES LIKE

[ajm188]

Overview of the Issue

When connected to a vtgate, running SHOW DATABASES shows all keyspaces the vtgate knows about. However when using SHOW DATABASES LIKE '%some_pattern%' the pattern is ignored completely, and the vtgate shows all keyspaces.

Reproduction Steps

Steps to reproduce this issue, example:

1. Create two keyspaces foo and bar
2. Run SHOW DATABASES LIKE 'foo'
3. View:

+----------------+
| Databases      |
+----------------+
| bar            |
| foo            |
+----------------+

Binary version

amason@workspace:~% vtgate --version
Version: 7200712c6 (Git branch 'HEAD') built on Wed May 13 09:48:15 PDT 2020 by amason@workspace using go1.14.2 linux/amd64

Operating system and Environment details

OS, Architecture, and any other information you can provide
about the environment.

amason@workspace:~% cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.6 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.6 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial
amason@workspace:~% uname -r
4.4.0-1105-aws
amason@workspace:~% uname -m
x86_64

[klkyy2020]

database in vitess means keyspace
show database command is same as show vitess_keyspace
maybe we can add a table named keyspace in _vt database in mysql, and change show databases like 'foo' to select * from _vt.keyspace where Database like 'foo';

[harshit-gangal]

Closed via #6750