[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`.

从 ServiceClientCommentComposer.java 中约第 221 行的 Client Overview 表开始，然后检查 JavaDocComment 以及 pull request 2280 和 2361 中的零散修复。确定哪些格式化职责应移入 JavaDocComment 的方法中，并验证所有表格元素的格式是否一致，而不依赖进一步的零散修复。

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