java: move known group id mapping into config default section

[zhumin8]

When adding new libraries, java currently have these hardcoded known group ids other than com.google.cloud:

librarian/internal/librarian/java/add.go

Lines 55 to 62 in 24b170e

	switch {
	case strings.HasPrefix(apiPath, "google/shopping/"):
	return setNonCloudMavenDefaults(lib, "com.google.shopping")
	case strings.HasPrefix(apiPath, "google/maps/"):
	return setNonCloudMavenDefaults(lib, "com.google.maps")
	case strings.HasPrefix(apiPath, "google/ads/"):
	return setNonCloudMavenDefaults(lib, "com.google.api-ads")
	}

We should convert these into config in the defaults for java:

librarian/internal/config/config.go

Line 221 in 24b170e

Java *JavaDefault `yaml:"java,omitempty"`

Preserve the mapping from API path to relevant group id.

This approach has benefits that:

• Supported group ids are visible from config
• When adding new group ids in the future, this is configurable and no changes to librarian needed