Serialize DataSourceType as plain string

[emschwartz]

Resolves FP-1046
Resolves FP-832

[linear]

FP-1046 Make Relay DataSourceType match schema

The generated API client expects the dataSourceType to be a string but the Relay returns it as an enum

[linear]

FP-832 Relay api & schema are out-of-sync

Something changed recently in the output for the /api/proxies/datasources endpoint. The type property of the datasource is no longer type string as the schema says, but an object with a single type property.

This is caused by a change in the proxy_types for the DataSourceType enum. Which adds tag = "type" to the serde/macro. definition