Add initial Decision (aka PDP) protos (#49875)

* Clone protos from fspmarshall/pdp-work-02

* Use decisionpb as the Go package alias

* Update DecisionService docs

* Split identity protos into separate files

* Update Metadata docs

* Split metadata into separate files

* Update Resource docs

* Update database access docs and fields

* Update SSH access docs and fields

* Ignore breaking changes on decision protos

* Update generated protos

* Use teleport.trait.v1.Trait

* Document mutual dependency between "ResourceID" types

* nit: s/ResourceID/ResourceId

* nit: Use the correct CamelCase field name in comments

* Update generated protos

* Add RouteToApp.target_port

* Update generated protos

* Update comments on TLSIdentity / tlsca.Identity

* Update generated protos