feat: add pg_textsearch extension

[ImSingee]

Close #83

[ImSingee]

@gbartolini The extension named pg_textsearch. But if I set metadata.name to this, an error will occur on test:

Cluster.postgresql.cnpg.io "pg_textsearch" is invalid: [metadata.name: Invalid value: "pg_textsearch": a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*'), spec.postgresql.extensions[0].name: Invalid value: "pg_textsearch": spec.postgresql.extensions[0].name in body should match '^[a-z0-9]([-a-z0-9]*[a-z0-9])?$']

Should I change the extension name to pgtextsearch (and use pg_textsearch as sql_name)? Or is it the test's responsibility to automatically use the name pg-textsearch rather than pg_textsearch for k8s-related names?

[ImSingee]

I think the second option is better (because pgtextsearch name is so ugly). So I created #87