Add extension to contribute test-resources settings to Micronaut plugin.

[wetted]

Need the ability to add something like the following to project generation, where a feature might need some extra test-resources setup to work.

For example using Cassandra and Test-resources together requires the following:

micronaut {
    testResources {
        clientTimeout = 600
    }
}

see #2113

[timyates]

Weirdly, I don't need this for cassandra 🤔