Admin putFunctionState REST API MediaType is multipart/form-data

**Describe the bug**
In https://github.com/apache/pulsar/blob/master/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/FunctionsBase.java#L535
MediaType is multipart/form-data, which is not expected since user won't upload file.

**Expected behavior**
MediaType should be application/json

BTW, the document about it is really poor, I can't successfully send any putFunctionState REST API request!

Start in pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/FunctionsBase.java around line 535 and inspect the putFunctionState REST endpoint declaration. Review the related API documentation mentioned in the report; done means the endpoint advertises application/json and its request usage is clear enough to send successfully without a multipart upload.

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