[sdk-platform-java] Support `:**` for wildcard matching in routing parameters

Storage has a need to support `:**` in the end of a routing parameter, see [proto config](https://github.com/googleapis/googleapis/blob/533398389c34b567e3ca11775fa127a111258ea1/google/storage/v2/storage.proto#L255). This was first found in [Ruby](https://b.corp.google.com/issues/277650689), but Java does not support it either after manual testing.
The original design of routing proto is not clear if this format should be supported or not. See [AIP-4222](https://google.aip.dev/client-libraries/4222) and [routing.proto](https://github.com/googleapis/googleapis/blob/533398389c34b567e3ca11775fa127a111258ea1/google/api/routing.proto) for details. There is an open [PR](https://github.com/aip-dev/google.aip.dev/pull/1067/files) to update the AIP to make it clear.
The current implementation in [PathTemplate](https://github.com/googleapis/gapic-generator-java/blob/main/api-common-java/src/main/java/com/google/api/pathtemplate/PathTemplate.java) only support `**` after `\`, we need to treat `:` as a proper delimeter as well.

从 api-common-java/src/main/java/com/google/api/pathtemplate/PathTemplate.java 开始，检查当前实现如何处理 `\\` 后的 `**`。将该行为与 storage proto 示例和 AIP-4222 进行比较；当 Java 按 routing format 的要求支持 routing parameter 末尾的 `:**` 时，即表示完成。

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