Merging to release-5: [DX-1081] Add last_updated to SessionState prot…

…obuf in the plugins data structures page (#4329)

[DX-1081] Add last_updated to SessionState protobuf in the plugins data structures page  (#4329)

* Add last_updated to SessionState protobuf message

tyk-docs/content/plugins/supported-languages/rich-plugins/rich-plugins-data-structures.md

@@ -248,6 +248,9 @@ Tags are embedded into analytics data when the request completes. If a policy ha
 `alias`
 As of v2.1, an Alias offers a way to identify a token in a more human-readable manner, add an Alias to a token in order to have the data transferred into Analytics later on so you can track both hashed and un-hashed tokens to a meaningful identifier that doesn't expose the security of the underlying token.
 
+`last_updated`
+A UNIX timestamp that represents the time the session was last updated. Applicable to *Post*, *PostAuth* and *Response* plugins. When developing *CustomAuth* plugins this should also be added to the SessionState instance.
+
 `id_extractor_deadline`
 This is a UNIX timestamp that signifies when a cached key or ID will expire. This relates to custom authentication, where authenticated keys can be cached to save repeated requests to the gRPC server. See [id_extractor]({{< ref "plugins/plugin-types/auth-plugins/id-extractor" >}}) and [Auth Plugins]({{< ref "plugins/plugin-types/auth-plugins/auth-plugins" >}}) for additional information.