Support raising an Error if table schema for a table is not known

What does this MR do and why?

The method Gitlab::Database::GitlabSchema.table_schema(<table_name>) tries to figure out to which table schema the table belongs: main or ci. It is using the database dictionary db/docs and some sane defaults and returns one of :gitlab_internal :gitlab_main, :gitlab_ci, :gitlab_shared

If this method fails to find a table schema, it will return a symbol, for example:

[2] pry(main)> Gitlab::Database::GitlabSchema.table_schema("new_table_that_was_not_in_data_directory")
=> :undefined_new_table_that_was_not_in_data_directory

After merge of !99287 (merged), it is now possible to specify the return value of table_schema method: it can be nil or the old behaviour :undefined_<table_name>.

This MR will add a table_schema! method: it will raise an error in case a schema is not found. This way, developers are aware that a newly created table should be added to the database directory db/docs

Apart from that, this MR will also support specifying the desired behaviour of table_schemas method: return nil or undefined_<table_name>.

Impact of this change

I ran git grep .table_schema and manually removed irrelavant matches. This leaves us with this list of calls to Gitlab::Database::GitlabSchema.table_schema and Gitlab::Database::GitlabSchema.table_schemas. For each of this usages, I checked if should allow nil or if we should raise an Error and explained my motivation

method table_schema

File	change?	Reasoning
app/models/concerns/cross_database_modification.rb	no change	Behaviour will not change
lib/gitlab/database/loose_foreign_keys.rb	raise an error	Developers should not create loose foreign keys for undefined schemas so let's raise an error.
scripts/decomposition/generate-loose-foreign-key	raise an error	This should fail if a table does not map to a schema

method table_schemas

method Gitlab::Database::GitlabSchema.table_schemas is looping over a list of tables and calls table_schema method on each of them. The default value of undefined parameter for table method will be used in this method as well.

File	change?	Reasoning
lib/gitlab/database/query_analyzers/gitlab_schemas_metrics.rb	no change	no change needed
lib/gitlab/database/query_analyzers/gitlab_schemas_validate_connection.rb	no change	no change needed
lib/gitlab/database/query_analyzers/prevent_cross_database_modification.rb	no change	no change needed
lib/gitlab/database/query_analyzers/restrict_allowed_schemas.rb	no change	no change needed

Screenshots or screen recordings

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How to set up and validate locally

1. Create a new table CREATE TABLE public.things (id serial primary key);
2. GitlabSchemasValidateConnection: In Rails Console:
   1. Thread.current[:query_analyzer_enabled_analyzers].append(::Gitlab::Database::QueryAnalyzers::GitlabSchemasValidateConnection)
   2. Project.joins("inner join things on 1=1")
   3. This should raise an error, having unknown_schema in it: #<Gitlab::Database::QueryAnalyzers::GitlabSchemasValidateConnection::CrossSchemaAccessError: The query tried to access ["projects", "things"] (of gitlab_main,unknown_schema) which is outside of allowed schemas ([:gitlab_main, :gitlab_shared, :gitlab_internal]) for the current connection 'main'>
   4. Project.joins(:users) will not raise an error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #381499 (closed)