Fix JsonMappingException when defaultRealm property is added in polaris-server.yml

[cgpoh]

When the defaultRealm property is defined before the realmContextResolver property in the polaris-server.yml file, for example:

defaultRealm: polaris

realmContextResolver:
  type: default

the following exception is thrown:

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Cannot invoke "org.apache.polaris.service.context.RealmContextResolver.setDefaultRealm(String)" because "this.realmContextResolver" is null (through reference chain: org.apache.polaris.service.dropwizard.config.PolarisApplicationConfig["defaultRealm"])

This occurs because of the following line in the setDefaultRealm method:

realmContextResolver.setDefaultRealm(defaultRealm);

At the time defaultRealm is set, realmContextResolver has not yet been initialized, causing the exception.

[adutra]

This probably fixes #562.

[cgpoh]

Thanks @adutra