Add issuable resource links DB table and rails model

What does this MR do and why?

Adds DB table issuable_resource_links and model for the table.

Adds specs for the model

Technical discussions for adding this table and model took place in tech discussion issue.

DB Migrations

db:migrate

== 20220525082303 CreateIssuableResourceLinks: migrating ======================
-- create_table(:issuable_resource_links, {})
-- quote_column_name(:link_text)
   -> 0.0000s
-- quote_column_name(:link)
   -> 0.0000s
   -> 0.0238s
== 20220525082303 CreateIssuableResourceLinks: migrated (0.0239s) =============

db:rollback VERSION=20220525082303

== 20220525082303 CreateIssuableResourceLinks: reverting ======================
-- drop_table(:issuable_resource_links, {})
   -> 0.0043s
== 20220525082303 CreateIssuableResourceLinks: reverted (0.0057s) =============

Screenshots or screen recordings

NA

How to set up and validate locally

NA

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 #363337 (closed)