Replace app_config_get_sync_client_config with _set_sync_client_config

[nirinchev]

What, How & Why?

It appears app_config_get_sync_client_config was never implemented and even if it was, it's not clear to me how it was supposed to be used. We do have a method to create a new sync client config already, so I figured we probably want to use that and then set the client config on the app config, but it's possible the original intention was different. Opening this as a draft to clarify things and will clean it up for review after we're aligned on the expected API.

☑️ ToDos

• [ ] 📝 Changelog update
• [ ] 🚦 Tests (or not relevant)
• [ ] C-API, if public C++ API changed
• [ ] bindgen/spec.yml, if public C++ API changed

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build nikola.irinchev_478

Details

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 1872 unchanged lines in 57 files lost coverage.
• Overall coverage decreased (-0.2%) to 90.821%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/realm/object-store/c_api/app.cpp	0	3	0.0%
<!--	Total:	0	3

Files with Coverage Reduction	New Missed Lines	%
src/realm/sync/noinst/server/server_history.cpp	1	63.7%
src/realm/util/flat_map.hpp	1	98.89%
test/fuzz_tester.hpp	1	57.73%
test/test_compaction.cpp	1	98.35%
test/test_index_string.cpp	1	93.48%
src/realm/array_integer_tpl.hpp	2	72.6%
src/realm/cluster.cpp	2	75.6%
src/realm/mixed.cpp	2	86.46%
src/realm/node_header.hpp	2	92.86%
src/realm/sync/config.hpp	2	0.0%
<!--	Total:	1872

Totals
Change from base Build 2405:	-0.2%
Covered Lines:	214568
Relevant Lines:	236253

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💛 - Coveralls

[rorbech]

This was resolved by https://github.com/realm/realm-core/pull/7891