Add support to .graphqls in addition to .graphql

[rgaiacs]

From https://docs.spring.io/spring-graphql/reference/request-execution.html#execution.graphqlsource.schema-resources

By default, the Boot starter looks for schema files with extensions ".graphqls" or ".gqls" under the location classpath:graphql/**, which is typically src/main/resources/graphql.

This PR adds ´.graphqls´ and ´.gqls´ to the list of supported files.

[carmenbianca]

Thanks @rgaiacs for the contribution! It looks great, merging.