feat(transform): Add Multi-Transform Support to Meta Transforms

[jshlbrd]

Description

• Adds Transforms field to all meta transform functions
• Deprecates Transform field in all meta transform functions
• Deprecates the MetaPipeline transform function

Motivation and Context

This simplifies the project's use of transform functions by making it so that every meta transform function accepts a list of transform functions (instead of only a single transform function). In v2.0 the existing transform support and the MetaPipeline transform will be removed.

For the existing transforms both transform and transforms fields have to be supported simultaneously to not introduce a breaking change, like this:

if len(tf.tfs) > 0 {
    msgs, err = Apply(ctx, tf.tfs, msg)
} else {
    msgs, err = tf.tf.Transform(ctx, msg)
}

Any new meta transform functions should only have a transforms field.

How Has This Been Tested?

• Added unit tests.
• Tested each updated example.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.