feat: add transformer to ViewColumnOptions

[tomwelsh11]

Description of change

Resolves #4339

This pull request adds a transformer field to ViewColumnOptions, to allow a view column to be unmarshalled by a custom ValueTransformer. This is the same functionality that currently exists for ColumnOptions and regular table entities. Note that you can already get the same result as this PR using @ts-ignore:

export const uppercase: ValueTransformer = {
    to: (entityValue: string) => {},
    from: (databaseValue: string) => databaseValue.toLocaleUpperCase()
};

@ViewEntity({ expression: `
    SELECT "post"."id" "id", "post"."name" AS "name", "category"."name" AS "categoryName"
    FROM "post" "post"
    LEFT JOIN "category" "category" ON "post"."categoryId" = "category"."id"
`})
export class PostCategory {
    @ViewColumn()
    id: number;

    @ViewColumn({ name: "name" })
    postName: string;

    //@ts-ignore
    @ViewColumn({ transformer: uppercase })
    categoryName: string;
}

However, it would be nice not to have to use @ts-ignore when needing this functionality.
I updated some of the existing tests to verify that this works correctly. Also added some documentation on the view column options.
This seemed like a bit of low hanging fruit that I could do - let me know if there's something I missed!

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

Thank you for contribution! 🎉