[sdk-platform-java] Add methods to JavadocComment to refactor how the Client Overview table is created

The Client Overview table in https://github.com/googleapis/sdk-platform-java/blob/main/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/ServiceClientCommentComposer.java#L221 has been brittle in terms of ensuring proper formatting of all of its elements. (See https://github.com/googleapis/sdk-platform-java/pull/2280 and https://github.com/googleapis/sdk-platform-java/pull/2361 as piecemeal fixes)

It would be better to refactor this table to ensure all elements are always properly formatted. Likely some new methods will need to be added to `JavaDocComment`.