Simplify `ValidateMultipartFormDataBody` and support optional fields

[LoicPoullain]

Issue

• The hooks does not support optional fields (issue: #1008).
• There is no error returned to the client when multiple files are uploaded whereas we expect a single one.
• The parameters passed to the hook are verbose even when uploading a simple file.
• We have a very large number of nested objects to access the files and fields in ctx: ctx.request.body.fields.name and ctx.request.body.files.avatar.
• The typing is not great in the controller method body because we have to "cast" variables: ctx.request.body.files.avatar as File|undefined.
• The name ValidateMultipartFormDataBody is not meaningful to someone reading the code: it shows the type of requests that are made but does not describe the really purpose of the hook.

New features

• Context.files is an instance of FileList which is a class to list Files(s). It has mainly two methods: ctx.files.push('avatar', file') and ctx.files.get('avatar') which always returns an array of Files(s) (that might be empty).
• New error MULTIPLE_FILES_NOT_ALLOWED (the value files.x is not replaced anymore).
• The interface of the hook has changed and accepts optional fields:

  @ParseAndValidateFiles(
    {
      profile: { required: true }
    },
    {
      type: 'object',
      properties: {
        description: { type: 'string' }
      },
      required: ['description'],
      additionalProperties: false
    }
  )
  

• ctx.files which is properly typed

Breaking changes

• ValidateMultipartFormDataBody is renamed ParseAndValidateFiles
• Code is compiled to es2019 which are supported by Node 14 and 16.
• File is exported from @foal/core and not @foal/storage anymore.
• ctx.request.body.fields becomes ctx.request.body.
• ctx.request.body.files.avatar as File becomes ctx.files.get('avatar')[0]
• ctx.request.body.files.avatars as File[] becomes ctx.files.get('avatars')
• The parameters of the hook have changed.
• Context.state and Context.request are readonly: they cannot be re-assigned.
• Previously saveTo: '' was regarded as an upload with buffer.

Dependencies

• @foal/storage
  • [email protected]
  • [email protected]

Checklist

• [x] Add/update/check docs (code comments and docs/ folder).
• [x] Add/update/check tests.
• [x] Update/check the cli generators.

[codecov-commenter]

Codecov Report

Merging #1048 (dad85c6) into v3-0-0 (120a7e0) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v3-0-0    #1048      +/-   ##
==========================================
+ Coverage   98.84%   98.86%   +0.01%     
==========================================
  Files          92       95       +3     
  Lines        1736     1763      +27     
  Branches      418      422       +4     
==========================================
+ Hits         1716     1743      +27     
  Misses         20       20              

Impacted Files	Coverage Δ
packages/core/src/common/file/file.ts	100.00% <ø> (ø)
packages/core/src/index.ts	0.00% <ø> (ø)
packages/core/src/common/file/file-list.ts	100.00% <100.00%> (ø)
packages/core/src/common/file/index.ts	100.00% <100.00%> (ø)
packages/core/src/common/index.ts	100.00% <100.00%> (ø)
packages/core/src/core/http/context.ts	100.00% <100.00%> (ø)

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