Add finding evidence header

What does this MR do?

This MR adds the Header model for Vulnerability Finding Evidence Requests and Responses. This is part of #239179.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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

== 20210506150833 CreateVulnerabilityFindingEvidenceHeaders: migrating ========
-- create_table(:vulnerability_finding_evidence_headers, {})
-- quote_column_name(:name)
   -> 0.0000s
-- quote_column_name(:value)
   -> 0.0000s
   -> 0.0134s
-- quote_table_name("check_01d21e8d92")
   -> 0.0000s
-- quote_table_name("check_3f9011f903")
   -> 0.0000s
-- quote_table_name(:vulnerability_finding_evidence_headers)
   -> 0.0000s
-- execute("ALTER TABLE \"vulnerability_finding_evidence_headers\"\nADD CONSTRAINT \"check_01d21e8d92\" CHECK (char_length(\"name\") <= 255),\nADD CONSTRAINT \"check_3f9011f903\" CHECK (char_length(\"value\") <= 8192)\n")
   -> 0.0013s
== 20210506150833 CreateVulnerabilityFindingEvidenceHeaders: migrated (0.0221s)

Rollback

== 20210506150833 CreateVulnerabilityFindingEvidenceHeaders: reverting ========
-- drop_table(:vulnerability_finding_evidence_headers)
   -> 0.0043s
== 20210506150833 CreateVulnerabilityFindingEvidenceHeaders: reverted (0.0176s)

Related to #239179