Add support for setting database and schema via environment variable

[bretwalker]

Are you a current Fivetran customer? Yes

What change(s) does this PR introduce? Adds the ability to set source database and schema via environment variable.

Did you update the CHANGELOG?

• [x] Yes

Does this PR introduce a breaking change?

• [ ] Yes (please provide breaking change details below.)
• [x] No (please provide an explanation as to how the change is non-breaking below.)

If fetching settings from environment variables fails, defaults will be returned, causing a fallback to the current var values in src_zendesk.yml.

Did you update the dbt_project.yml files with the version upgrade (please leverage standard semantic versioning)? (In both your main project and integration_tests)

• [x] Yes

Is this PR in response to a previously created Bug or Feature Request

• [x] Yes, Issue/Feature https://github.com/fivetran/dbt_zendesk_source/issues/33
• [ ] No

How did you test the PR changes?

• [ ] Buildkite
• [x] Local (please provide additional testing details below)

I tested by setting both the new var values (zendesk_schema_env_var and zendesk_database_env_var) and the current var values (zendesk_schema and zendesk_database) and running dbt build against a Snowflake warehouse.

Select which warehouse(s) were used to test the PR

• [ ] BigQuery
• [ ] Redshift
• [x] Snowflake
• [ ] Postgres
• [ ] Databricks
• [ ] Other (provide details below)

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