Extend the number of users we are able to create in memory

## Background
For the purpose of Load testing and maybe even other types of testing it would be good to be able to pull in-memory-users from some file (app.conf or other) where we can specify name, password and role.

Currently, we only create one in-memory-user for the purpose of the testing user/change me. It is pulled from app.conf and he has no standard ROLE_USER.

## Feature
Load in-memory-user more dynamically and be able to have more of them.

## Example [Optional]
```properties
menas.auth.inmemory.1.user=user
menas.auth.inmemory.1.password=changeme
menas.auth.inmemory.1.role=ROLE_USER
menas.auth.inmemory.2.user=user
menas.auth.inmemory.2.password=changeme
menas.auth.inmemory.2.role=ROLE_USER
menas.auth.inmemory.3.user=user
menas.auth.inmemory.3.password=changeme
menas.auth.inmemory.3.role=ROLE_USER
```
OR
Separate HOCON-like file
```HOCON
standard-user = { password = "aligator1", role = "ROLE_USER"
users = [
${standard-user} { name = "user1" },
${standard-user} { name = "user2" },
${standard-user} { name = "user3" },
{ name = "menas-admin", password = "aligator2", role = "ROLE_ADMIN" }
]
```

Start with the current app.conf settings and the code that creates the existing testing user. Compare the proposed property-based and HOCON-like configuration shapes, then define how multiple names, passwords, and roles should be loaded. Done means load testing can create more than one in-memory user with the configured credentials and roles, with coverage for the chosen configuration format.

Vom Indexierungsmodell aus dem Issue-Text verfasst.