Refactor namex-solr-api to use cloudsql connector 

Namex Solr API is using username+password to authenticate with cloudsql db in gcp:

https://github.com/bcgov/namex-search/tree/main/namex-solr-api

Need to rewrite it to use cloudsql db for IAM service account authentication, as most other apps/services/jobs in namex repo. 

E.g. see https://github.com/bcgov/namex/pull/1924