Add `package_manager` column to `sbom_occurrences` table

What does this MR do and why?

This is a denormalization to improve the performance of the group-level dependency list page.

Database review

rake db:migrate:up

main: == [advisory_lock_connection] object_id: 218840, pg_backend_pid: 97336
main: == 20230718120802 AddPackageManagerColumnToSbomOccurrences: migrating =========
main: -- transaction_open?()
main:    -> 0.0000s
main: -- add_column(:sbom_occurrences, :package_manager, :text, {:if_not_exists=>true})
main:    -> 0.1064s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- execute("ALTER TABLE sbom_occurrences\nADD CONSTRAINT check_3f2d2c7ffc\nCHECK ( char_length(package_manager) <= 255 )\nNOT VALID;\n")
main:    -> 0.0007s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- execute("ALTER TABLE sbom_occurrences VALIDATE CONSTRAINT check_3f2d2c7ffc;")
main:    -> 0.0006s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20230718120802 AddPackageManagerColumnToSbomOccurrences: migrated (0.1257s)

main: == [advisory_lock_connection] object_id: 218840, pg_backend_pid: 97336

rake db:migrate:down

main: == [advisory_lock_connection] object_id: 219920, pg_backend_pid: 96874
main: == 20230718120802 AddPackageManagerColumnToSbomOccurrences: reverting =========
main: -- remove_column(:sbom_occurrences, :package_manager, {:if_exists=>true})
main:    -> 0.0121s
main: == 20230718120802 AddPackageManagerColumnToSbomOccurrences: reverted (0.0152s)

main: == [advisory_lock_connection] object_id: 219920, pg_backend_pid: 96874

Related to Improve performance of `Sbom::DependenciesFinde... (#415665 - closed).

MR acceptance checklist

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