Add Finding Evidence Sources

What does this MR do?

This MR creates a new model for Vulnerability Finding Evidence Sources. 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 #267489

Screenshots or Screencasts (strongly suggested)

N/A

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

== 20210707151536 CreateVulnerabilityFindingEvidenceSources: migrating ========
-- create_table(:vulnerability_finding_evidence_sources, {})
-- quote_column_name(:name)
   -> 0.0001s
-- quote_column_name(:url)
   -> 0.0000s
   -> 0.0111s
-- quote_table_name("check_86b537ba1a")
   -> 0.0000s
-- quote_table_name("check_0fe01298d6")
   -> 0.0000s
-- quote_table_name(:vulnerability_finding_evidence_sources)
   -> 0.0000s
-- execute("ALTER TABLE \"vulnerability_finding_evidence_sources\"\nADD CONSTRAINT \"check_86b537ba1a\" CHECK (char_length(\"name\") <= 2048),\nADD CONSTRAINT \"check_0fe01298d6\" CHECK (char_length(\"url\") <= 2048)\n")
   -> 0.0018s
== 20210707151536 CreateVulnerabilityFindingEvidenceSources: migrated (0.0165s)

Rollback

== 20210707151536 CreateVulnerabilityFindingEvidenceSources: reverting ========
-- drop_table(:vulnerability_finding_evidence_sources)
   -> 0.0021s
== 20210707151536 CreateVulnerabilityFindingEvidenceSources: reverted (0.0191s)

Related to #267489