[32753 backport to 0.4.2] Lowercase the BridgeDistribution value from torrc in descriptors.

[ahf]

This patch ensures that we always lowercase the BridgeDistribution from
torrc in descriptors before submitting it.

See: https://bugs.torproject.org/32753

[coveralls]

Pull Request Test Coverage Report for Build 7900

• 5 of 6 (83.33%) changed or added relevant lines in 2 files are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.01%) to 62.28%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/feature/relay/router.c	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/feature/dirauth/shared_random.c	4	85.0%
src/feature/dirauth/dirvote.c	9	64.85%

Totals
Change from base Build 7800:	-0.01%
Covered Lines:	48485
Relevant Lines:	77850

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💛 - Coveralls