feat: centralized license evidence gatrhering

[jkowalleck]

Gathering license evidence is a common feature of downstream users.
As a library, this should be centralized in here, so that maintenance efforts are minimized.

currently known implementations downstream:

• for webpack: https://github.com/CycloneDX/cyclonedx-webpack-plugin/blob/72700f06d00eac79fa3b91fe838bd78c583346a2/src/extractor.ts#L133-L173
• for yarn: upcoming via Extraction of license text from files. cyclonedx-node-yarn#193
• for npm via feat: Add license text as evidence cyclonedx-node-npm#1243

related downstream issues

• Reconsider license name patterns cyclonedx-node-yarn#229

---

Requirements

license for evidence

• have no acknowledgement -- they are observed
• have a name that expresses their origin -- file: ${filePath}
• have the full license texts as attachment
  • are base64 encoded, when auto-detected, just to be save
  • have an appropriate content type of text/...

files to take into account

• /^LICEN[CS]E/i -- general file pattern
• /.LICEN[CS]E$/i -- common file pattern for multi-license projects
• /^NOTICE$/ -- file with this exact name is relevant in terms of Apache-2.0 license

text content type

like so

const MAP_TEXT_EXTENSION_MIMETYPE: Readonly<Record<string, MimeType>> = {
  '': 'text/plain', // our scope is text!
  '.csv': 'text/csv',
  '.htm': 'text/html',
  '.html': 'text/html',
  '.licence': 'text/plain',
  '.license': 'text/plain',
  '.md': 'text/markdown',
  '.rst': 'text/prs.fallenstein.rst',
  '.txt': 'text/plain',
  '.xml': 'text/xml' // not `application/xml` -- our scope is text!
} as const

additional license-file related file extensions

• .apache
• .apache2 <<- missing in most implementations
• .bsd
• .mit
• .gpl

...

to be continued

[jkowalleck]

Previous attempts to solve this were postponed, since there is not enough known about the requirements downstream.
Any API released today would probably be insufficient - it would require extending a lot or be unused.
Lets wait until more specialized downstream users have solved this problem.