Fix bug with metadata-type not work since 2.7.5 describe in issue #5667

[laddcn]

What is the purpose of the change

Fix bug with metadata-type not work since 2.7.5 describe in issue #5667

Brief changelog

1. Add METADATA_TYPE_KEY in CommonConstants
2. When getting WritableMetadataService, "metadata-type" and "metadata" are both supported for compatible.

Verifying this change

1. Before this PR, the following config does not work
   <dubbo:application name="demo-provider" metadata-type="remote"/>
2. After this PR, metadata are published to metadata center.

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Run mvn clean install -DskipTests=false & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
• If this contribution is large, please follow the Software Donation Guide.