feat: Adds `stored_source` attribute to `mongodbatlas_search_index` resource and corresponding data sources

Description

feat: Adds stored_source attribute to mongodbatlas_search_index resource and corresponding data sources

PR blocked until stored_source updated can be implemented after CLOUDP-260196 is resolved

To-do list in the PR:

Implement update when possible
Update doc
Update examples
Unit tests for functions in model file (expanders, flatteners, marshalers and unmarshalers)

Link to any related issue(s): CLOUDP-215071

[ ] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
[x] New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
[ ] This change requires a documentation update
[ ] Documentation fix/enhancement

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[x] I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
[x] I have added any necessary documentation (if appropriate)
[x] I have run make fmt and formatted my code
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