Add PostgresSequence view

Ref: #373887 (closed)

What does this MR do and why?

This MR adds a new posgres view which is used to identify the sequence name of a given table.

Gitlab::Database::PostgresSequence.by_table_name('ci_job_artifacts')
=> [#<Gitlab::Database::PostgresSequence:0x00000001614bc690 seq_name: "ci_job_artifacts_id_seq", table_name: "ci_job_artifacts", col_name: "id", object_kind: "S">]

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