[core] Support text/plain content type for REST catalog request body

[ArnavBalyan]

Purpose

• REST create table fails because post and delete are sent without a content type, defaulting to plain text, which JSON expecting REST servers reject before processing.
• Fix by setting the request body content type to json in the http client, and by body type in the simple client, (form encoded for form bodies and JSON otherwise).
• Closes user reported bug [Bug] REST Catalog Content-Type Error with Paimon 1.3.1 #7092

Tests

• UT

[ArnavBalyan]

cc @JingsongLi @leaves12138 thanks! :)