Unique Index for Vulnerability LInks

What does this MR do?

This MR adds a unique key index for vulnerability links to prevent duplicate insertion

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Start on master branch.
2. Enable the finding link storage

   Feature.enable(:vulnerability_finding_replace_metadata)

3. Import https://gitlab.com/gitlab-examples/security/security-reports into your local environment.
4. Run the pipeline twice.
5. Find a vulnerability that has links, usually a Dependency Scanning vulnerability will have some.
6. Confirm that there are 2 links for the vulnerability finding

   Vulnerability.find(<vulnerability_id>).finding.finding_links.count

7. Clear findings table

   Vulnerabilities::FindingLink.all.each { |l| l.destroy }

8. Go to the 330882-dependency-scanning-vulnerability-reports-a-lot-of-duplicate-links-index branch and run the database migration (rails db:migrate)
9. Run the pipeline twice.
10. Confirm that there is only 1 link for the vulnerability finding

    Vulnerability.find(<vulnerability_id>).finding.finding_links.count

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Database

Migration

== 20210811193033 AddUniqueIndexToVulnerabilityFindingLinks: migrating ========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:vulnerability_finding_links, [:vulnerability_occurrence_id, :name, :url], {:unique=>true, :name=>"finding_link_name_url_idx", :algorithm=>:concurrently})
   -> 0.0039s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:vulnerability_finding_links, [:vulnerability_occurrence_id, :name, :url], {:unique=>true, :name=>"finding_link_name_url_idx", :algorithm=>:concurrently})
   -> 0.0033s
-- execute("RESET statement_timeout")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:vulnerability_finding_links, [:vulnerability_occurrence_id, :url], {:unique=>true, :where=>"name is null", :name=>"finding_link_url_idx", :algorithm=>:concurrently})
   -> 0.0017s
-- add_index(:vulnerability_finding_links, [:vulnerability_occurrence_id, :url], {:unique=>true, :where=>"name is null", :name=>"finding_link_url_idx", :algorithm=>:concurrently})
   -> 0.0020s
== 20210811193033 AddUniqueIndexToVulnerabilityFindingLinks: migrated (0.0162s) 

Rollback

== 20210811193033 AddUniqueIndexToVulnerabilityFindingLinks: reverting ========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:vulnerability_finding_links, [:vulnerability_occurrence_id, :name, :url], {:name=>"finding_link_name_url_idx", :algorithm=>:concurrently})
   -> 0.0031s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- remove_index(:vulnerability_finding_links, {:name=>"finding_link_name_url_idx", :algorithm=>:concurrently, :column=>[:vulnerability_occurrence_id, :name, :url]})
   -> 0.0082s
-- execute("RESET statement_timeout")
   -> 0.0006s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:vulnerability_finding_links, [:vulnerability_occurrence_id, :url], {:name=>"finding_link_url_idx", :algorithm=>:concurrently})
   -> 0.0015s
-- remove_index(:vulnerability_finding_links, {:name=>"finding_link_url_idx", :algorithm=>:concurrently, :column=>[:vulnerability_occurrence_id, :url]})
   -> 0.0032s
== 20210811193033 AddUniqueIndexToVulnerabilityFindingLinks: reverted (0.0248s) 

Related to #330882 (closed)