[NEOS-745] Update RPCs to specify idempotency

[nickzelei]

This allows them to be used as GET methods in Connect, as well as well as tells the SDKs that they can safely retry the method transparently if they are met with an error or GOAWAY.

More info here: https://buf.build/blog/introducing-connect-cacheable-rpcs

An example of how this is used:

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Can be then hit via: http://localhost:8080/mgmt.v1alpha1.TransformersService/GetSystemTransformers?encoding=json&message={}

_{From SyncLinear.com | NEOS-745}