Cherry-pick #15686 to 7.x: Remove datasource option from SQL module and add tests

[jsoriano]

Cherry-pick of PR #15686 to 7.x branch. Original message:

What does this PR do?

• Remove datasource option from SQL module. This option was intended to set the DSN of a database connection, and we were ignoring the hosts setting. In other SQL modules we are using the values in host as DSNs, do here the same for consistency. Host is redacted when we cannot parse it as it can contain passwords.
• StandardizeEvent is exposed in mbtest.Fetcher interface so we can more easily check contents of events.
• Add integration tests of the module with MySQL and PostgreSQL.
• Add real data.json with data from MySQL and PostgreSQL.

Why is it important?

• To have consistency between this new SQL module and other engine-specific SQL modules.
• To have automatic tests.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Continues [Metricbeat] Add SQL module (#13106) #13257
• Solves part of Generic SQL module #15048

[kaiyan-sheng]

There is no changelog in the original PR, is that ok?

[jsoriano]

There is no changelog in the original PR, is that ok?

Yep, this is a change in an unreleased feature.