add support for tagging if an endpoint is read or write

[sixinli]

it would be useful for consumer to know the potential side effects of the endpoints

[iamdanfox]

@sixinli I think my concern with adding this extra metadata is I don't know how we could make sure it was correct. I.e. what if an endpoint was annotated as READ_ONLY but then someone actually made the endpoint do a migration-on-read that involved copying some data to a new store. The annotation of 'read/write' becomes a bit fuzzy at this point, and I'm not sure what value can be derived from metadata that is not always accurate.

[sixinli]

thats a valid concern... its hard to enforce because people can technically do anything they want even with a GET endpoint... this is more just for good practice