Create set of functions for the filter evaluations

[AuHau]

govaluate allows adding custom functions that can be exposed to the filter expression. We could add all sort of convenience functions for example for:

• string manipulations
  • contains() - bool if contains substring
  • startsWith() - bool if string starts with substring
  • endsWith() - bool if string ends with substring

[AuHau]

Now that we have switched to expr the previously mentioned functions are actually already implemented there, but this is still possible and we should think of what custom functions would make sense to include.

[AuHau]

• https://github.com/antonmedv/expr/blob/master/docs/Language-Definition.md
• https://github.com/antonmedv/expr/blob/master/docs/Custom-Functions.md