Add FindingEvidenceRequest model

What does this MR do?

This MR creates a new model for Vulnerability Finding Evidence Requests. This model will be replacing metadata in the finding. We are not using this table yet, that will be handled by a future MR. Related to #239179

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Database

Migrate

rails db:migrate
== 20210415172516 CreateVulnerabilityFindingEvidenceRequests: migrating =======
-- create_table(:vulnerability_finding_evidence_requests, {})
-- quote_column_name(:method)
   -> 0.0000s
-- quote_column_name(:url)
   -> 0.0000s
   -> 0.0123s
-- quote_table_name("check_d9d11300f4")
   -> 0.0000s
-- quote_table_name("check_8152fbb236")
   -> 0.0000s
-- quote_table_name(:vulnerability_finding_evidence_requests)
   -> 0.0000s
-- execute("ALTER TABLE \"vulnerability_finding_evidence_requests\"\nADD CONSTRAINT \"check_d9d11300f4\" CHECK (char_length(\"method\") <= 32),\nADD CONSTRAINT \"check_8152fbb236\" CHECK (char_length(\"url\") <= 2048)\n")
   -> 0.0009s
== 20210415172516 CreateVulnerabilityFindingEvidenceRequests: migrated (0.0253s)

Rollback

rails db:rollback
== 20210415172516 CreateVulnerabilityFindingEvidenceRequests: reverting =======
-- drop_table(:vulnerability_finding_evidence_requests)
   -> 0.0055s
== 20210415172516 CreateVulnerabilityFindingEvidenceRequests: reverted (0.0199s)

Related to #239179