Allow user to specify buffer by adding `cbor.MarshalToBuffer()`, `UserBufferEncMode` interface, etc.

[fxamacker]

Currently, the encoder uses a built-in buffer pool. This PR allows encoding to a user specified buffer rather than using the built-in buffer pool.

This PR wraps and uses a function implemented in PR #521 by @benluddy. Thanks Ben! :+1:

This PR adds:

• cbor.MarshalToBuffer() uses codec's default options to encode to user provided buffer instead of using built-in buffer pool.

• UserBufferEncMode interface extends EncMode interface with MarshalToBuffer() so user can provide buffer for encoding instead of using built-in buffer pool.

• EncOptions.UserBufferEncMode() returns UserBufferEncMode

• EncOptions.UserBufferEncModeWithTags() returns UserBufferEncMode

• EncOptions.UserBufferEncModeWithSharedTags() returns UserBufferEncMode

This PR added a check to return error if user provided buffer *bytes.Buffer is nil.

[fxamacker]

@benluddy @x448 PTAL 🙏