Allow setting Malli transformers that apply to all body formats

[FelipeCortez]

It may be useful to use decode in coercions for body formats besides JSON:

reitit/modules/reitit-malli/src/reitit/coercion/malli.cljc

Lines 111 to 116 in e204d4f

	(def default-options
	{:transformers {:body {:default default-transformer-provider
	:formats {"application/json" json-transformer-provider}}
	:string {:default string-transformer-provider}
	:response {:default default-transformer-provider
	:formats {"application/json" json-transformer-provider}}}

For more details, see Slack threads here and here.