feat: pass ClassTransformOptions to the transformFn of @Transform

[lucas-gregoire]

The goal of this PR is to be able to access to ClassTransformOptions provided by transformation methods directly inside the transformFn of @Transform decorators.

class User {
    @Expose()
    name: string;

    @Expose()
    @Transform((_0, _1, _2, options) => options.groups)
    groups: string[];
}

plainToClass(
    User,
    { name: 'Johny Cage' },
    { groups: ['user.full'] },
)

// Result
// User {
//    name: 'Johny Cage',
//    groups: ['user.full'],
// }

Another use-case could be:
If i want to use a transformation method (e.g. plainToClass) inside a @Transform and extend the groups, I can do:

class User {
    @Expose()
    name: string;

    @Expose()
    @Transform(
        (value, _0, _1, { groups: inheritedGroups }) => plainToClass(
            Foo, 
            value, 
            { groups: [...inheritedGroups, 'additional_group'] }
        ),
        { toClassOnly: true }
    )
    foos: Foo[];
}

This pull request fixes following issues:

• feat: include custom option keys as argument in @Transform #238
• feat: include group as argument in @Transform #208

[cojack]

Hmm can we trigger somehow travis to make a run once again for this MR, to check why the heck tests fail?

[lucas-gregoire]

I force-pushed the last commit to trigger a new build.
But there is a compatibility issue between gulp@3 and node@12 (current stable): gulpjs/gulp#2324
So it's necessary to upgrade gulp to v4

[lucas-gregoire]

#266
#263

[stewartmcgown]

Is something blocking this PR being merged? Seems like a very useful feature. Happy to review if one is needed.

[julien-sarazin]

Hi there, I need this feature so hard. Please make class-transformer great again!

[NoNameProvided]

This is related to #330. We need to rebase this after that one is merged.

[lucas-gregoire]

Rebase is done, fell free to comment/review 😉

[github-actions]

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