Move bson files to core utils

[jedelbo]

What, How & Why?

The bson library should be available to core/core as well

☑️ ToDos

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

[coveralls-official]

Pull Request Test Coverage Report for Build github_pull_request_285276

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 892 unchanged lines in 35 files lost coverage.
• Overall coverage decreased (-0.01%) to 91.644%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/util/compression.cpp	1	91.59%
src/realm/array_string.cpp	2	88.0%
src/realm/sync/subscriptions.hpp	2	16.13%
src/realm/sort_descriptor.cpp	3	93.7%
src/realm/sync/network/network.cpp	3	89.66%
src/realm/sync/noinst/protocol_codec.hpp	3	76.21%
test/object-store/util/test_file.hpp	3	94.83%
test/test_util_network.cpp	3	97.2%
test/object-store/c_api/c_api.cpp	4	99.48%
test/object-store/util/sync/sync_test_utils.hpp	4	76.32%

Totals
Change from base Build 1852:	-0.01%
Covered Lines:	230757
Relevant Lines:	251796

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💛 - Coveralls