Define span links in yaml definition

[thompson-tomo]

Is your feature request related to a problem? Please describe.
I want my definitions of spans to also indicate what links may/should/must have

Describe the solution you'd like
Introduction of a new array property on the span definition called links which is an array of objects.

Each object would contain the following:

• Ref: what type of span it should be linked to & is required
• brief/note
• Attributes: defaults to empty list and can only be defined if link type is other
• link type: parent, child or other which is default

Describe alternatives you've considered

Defining links as first class signal and re-using but limited benefit especially with attribute groups available

Additional context

• Semantic convention for span link names semantic-conventions#1057

[jsuereth]

We think we can add span links to V2 schema in a non-breaking way going forward so we're going to defer this work until after V2 lands.