Improve user auth in gafferpop

[t92549]

Describe the new feature you'd like
There is basic auth in Gaffer's tinkerpop graph provided via config:

Gaffer/library/tinkerpop/src/main/java/uk/gov/gchq/gaffer/tinkerpop/GafferPopGraph.java

Line 141 in 920bbc3

.userId(configuration().getString(USER_ID, User.UNKNOWN_USER_ID))

We should look into how this might actually work with real auth, for example using a provided user factory

Describe alternatives you've considered
If connecting to a proxy store, could auth be handled by the target store?

[tb06904]

Some discussion about this in: Originally posted by @tb06904 in #3195 (comment)

It appears the gremlin server already provides a kind of authentication layer for users to authenticate against before accessing the server see: https://tinkerpop.apache.org/docs/3.7.2/reference/#security

A proposal would be to utilise this layer instead of a user factory to either reuse or create a custom Authenticator class that can handle the initial user auth and then an Authorizer that configures any additional restrictions based on the supplied query and handle modifying the traversal strategy of the request to add user object or user ID. An example of an Authorizer can be found here.