Allow to specify source database instead of defaulting to bronze database in __get_silver_dataflow_spec_dataframe

[rtdtwo]

Issue

The FAQs describes that any data source can be used for silver tables. However, the code always tries to find the bronze database from the onboarding rows, and there's no explicit mention of a source database other than a bronze database.

Fix

Change the code to accomplish the following:

• Use bronze_database_{env} if the field is found in onboarding rows.
• If not, find and use source_details.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (2a93dd9) 90.28% compared to head (f4fbd2b) 89.95%.

Files	Patch %	Lines
src/onboard_dataflowspec.py	40.00%	1 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #34 +/- ## ========================================== - Coverage 90.28% 89.95% -0.34% ========================================== Files 8 8 Lines 803 806 +3 Branches 149 151 +2 ========================================== Hits 725 725 - Misses 31 32 +1 - Partials 47 49 +2 ``` | [Flag](https://app.codecov.io/gh/databrickslabs/dlt-meta/pull/34/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=databrickslabs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/databrickslabs/dlt-meta/pull/34/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=databrickslabs) | `89.95% <40.00%> (-0.34%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=databrickslabs#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ravi-databricks]

@rtdtwo, Needs to add unit tests for added new lines as per above code coverage report.