Added YAML en(de)coding support to MessageCard

[pcanilho]

Current (non-)issue

I am planning to use this library with pre-built templates, provided by users, in YAML format. Due to the type of the ValidateFunc field of MessageCard, YAML decoding/encoding doesn't work out-of-the-box since the marshaller attempts to parse that field.

	// ValidateFunc is an optional user-specified validation function that is
	// responsible for validating a MessageCard. If not specified, default
	// validation is performed.
	ValidateFunc func() error `json:"-"`

Furthermore, other fields such as Type and Context require that an extra @ symbol is prefixed when encoding its value. Without effectively providing YAML tags, this is not feasible without re-implementing the structs in the client side.

Goal

With this PR, clients of this library will be able to decode/encode the structs found within the messagecards.go file into YAML format out-of-the-box with no impact to existing usage.

Impact

No impact is expected since no new libraries need to be included in this library nor does the inclusion of new tags affect the overall behaviour of the codebase.

[atc0005]

@pcanilho Thank you for submitting this PR.

I'll look over it and get back to you when I can. It may be a few days before you hear from me.

The linter errors are for existing changes (my fault), so please disregard them.

[atc0005]

@pcanilho Thank you again for submitting this PR.

There are some further cleanup tasks I'd like to have applied before the next stable release, but I'll go ahead and tag another alpha release in case you want to reference your changes (e.g., go.mod) in the meantime.

No timeline on the next stable release, but I'm hoping for sooner rather than later.

[pcanilho]

Hi @atc0005 ,
Thank you for your prompt responses and for the new tag creation, this is deeply appreciated as it unblocks me. 🙂