Add pattern definition rules

[arey]

This development follows on from the Pull Request spring-petclinic/spring-petclinic-angular#142 from @Holger-Mayer. I quote:

An anomaly is defined within the openAPI validation.
We define some fields with a minimal length but we do not have a pattern validation rule defined. This enables the user of the api to define i.e. names with all blank characters. Here in the future, the openApi description should define pattern validation rules.
In the meantime, it is assumed, that any required name field with a length > 0 may not start with a character other than [A-za-z]. This is implemented in this pull request.

We could add some validation rules similar to the spring-petclinic-angular. Before starting this issue, we have to list them here.