feat: populate pg-meta crypto key in pg-meta and studio

[mattrossman]

Draft to go along with supabase/supabase#39041

• Adds PgmetaCryptoKey to the studio config, defaulting to "SAMPLE_KEY" to match the postgres-meta fallback
• Populates studio's PG_META_CRYPTO_KEY and pg-meta's CRYPTO_KEY with this

[coveralls]

Pull Request Test Coverage Report for Build 18174729785

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 54.671%

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Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 18154870716:	-0.02%
Covered Lines:	6391
Relevant Lines:	11690

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💛 - Coveralls

[mattrossman]

I figure PgmetaCryptoKey belongs here since PgmetaImage is already there, but was wondering if it's better to split the pgmeta stuff into its own struct.