Default values of filters should support placeholders like "$current_user"

[sleidig]

As a admin I want to filter lists to match only records that link the current logged in user, to make the interface as simple as possible for users.

Entity lists can display filters above the table to let users narrow down the results. These are configured in the overall system config (Config:CONFIG_ENTITY doc in database / config-fix.ts demo config). We can already define a default value for a filter, to pre-select and pre-filter automatically (e.g. filters: [{ "id": "authors", "default": "User:ff8b8bf8-ca1b-4ea4-82ff-4491d1c4ee06" }]). This system should also suppport placeholders, like we use them for entity field defaultValue definitions.

e.g. support a FilterConfig definition like this:

{
  "id": "authors",
  "default": "$current_user"
}

(this should support all "dynamic" default values, i.e. also "$now")

Proposed Solutions & Alternatives
Reuse the implementation of the DynamicPlaceholderValueService and integrate this into the FilterGeneratorService.
Write unit tests first and develop this following the TDD approach.