Add com.snowplowanalytics.snowplow/button_click/jsonschema/1-0-1

[Jack-Keene]

This PR makes the label property of the button_click schema an optional property. This is to handle buttons without any inner text - for example icon buttons.

I've also added the $supersedes property for the previous v1-0-0 schema

[istreeter]

Hi @Jack-Keene regarding $supersedes, please refer to this conversation in a previous PR. We concluded that in Iglu Central you need to add $supersedes to the new schema AND $supersededBy to the old schema.

After you've read that, then please refer to this follow-up conversation where it got decided simply to remove the required from the previous schema, instead of releasing a new schema.

I think the situation for button_click is the same as the situation for client_session which got discussed in the earlier conversations.

[matus-tomlein]

Sorry missed this PR. Agree with @istreeter, but just want to add that when in the client_session case we replaced the schema, a customer complained that it "broke a bunch of events for them". I don't really understand how, @johnmicahreid followed up with them to get more info. But I think it might be better to avoid the replace and just release a new patch version that we update in our trackers. It's not as neat as customers will need to update their tracking, but probably safer.

[istreeter]

Oh... so that might be because in Redshift it is not possible to remove a not null constraint from an existing column. In which case this change needs to be a 2-0-0 schema, not a 1-0-1 schema.

[matus-tomlein]

hmm... @johnmicahreid do we still want the change (make the label optional) if it requires a new major version of the schema (so a new column in the events table or a new table in redshift which could be breaking for downstream applications)?