Add method to ensure a Perl version exists in PerlVersion

[preaction]

Right now, the correct way to ensure a Perl version exists in the perl_version table is to do:

my $result = $schema->resultset( 'PerlVersion' )->find_or_create({ version => '5.26.0' });

This is less than optimal for two reasons:

1. We often don't care to get the result object back, we just need to ensure that the Perl version exists in the database.
2. This code requires the reader to know that the version field is parsed to find all the other information in the perl_version table. If some other field is used instead, like perl, the data will not be correct.

It would be better if we had a method named ensure_exists that takes a single argument, the Perl version string. It would be called like this:

$schema->resultset( 'PerlVersion' )->ensure_exists( '5.26.0' );

This reads better, and we can improve performance slightly inside by not creating a result object.