Allow single existing secret for tooljet server deployment or existing secret option for tooljetdb_external_postgresql

[iamvtor]

There are configuration options in values.yaml for postgresql and external_postgresql, but not for tooljetdb_external_postgresql.

Since all of these configuration options are only used by the tooljet server deployment, an option to use an existing secret instead of creating one would be a good solution to define all environment variables in one place.

Alternatively, adding an existing secret option for tooljetdb_external_postgresql also makes sense.

[iamvtor]

On another note, the postgrest deployment also doesn't allow existing secret configuration. The values for postgrest are sensitive and shouldn't be exposed in values.