Add support for sequences for database validation schema

What does this MR do and why?

This adds basic support for identifying missing database sequences.

This does not yet:

• Check ownership of sequences
• Check that columns have properly been assigned to the right sequence.

References

Relates to #396878

How to set up and validate locally

mysql-17.9.2.sql

In bin/rails console, I ran:

bad_sql = Gitlab::Schema::Validation::Sources::StructureSql.new('/tmp/mysql-17.9.2.sql')
db = Gitlab::Schema::Validation::Sources::Database.new(ActiveRecord::Base.connection)
validators = [Gitlab::Schema::Validation::Validators::MissingSequences]
diffs = Gitlab::Schema::Validation::Runner.new(db, bad_sql, validators: validators).execute
diffs.map(&:error_message)

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