Integrate registry feature into rpk CLI

[vuldin]

It would make many tasks easier if rpk could interact with the schema registry. Some commands it could perform:

1. Uploading a schema found in file schemas/avro/market_activity.avsc to a subject avro-market-activity-value, which implies it is the schema for the value of the message in topic avro-market-activity. The subject is created if it doesn't already exist. If it does exist, then the schema is added as the latest version for that subject. In the below example, this schema is the 4th schema to be uploaded globally, and the 2nd schema uploaded to the subject avro-market-activity-value:

> rpk registry schema add --subject avro-market-activity-value schemas/market_activity.avsc
ID    SUBJECT                       VERSION
4     avro-market-activity-value    2

2. Similar to above, but for a protobuf schema. This is the 5th schema registered globally and the 1st for the subject proto-market-activity-value:

> rpk registry schema add --subject proto-market-activity-value schemas/market_activity.proto
ID    SUBJECT                       VERSION
5     proto-market-activity-value    1

3. List subjects:

> rpk registry subject list
NAME                          SCHEMA_VERSIONS    TOPICS
avro-market-activity-value    2                  avro-market-activity
proto-market-activity-value   1                  proto-market-activity

4. Create a subject new-subject and associate it with two existing topics (trying this with non-existent topics could be an error?). The subject is new so doesn't have any schemas yet:

> rpk registry subject create new-subject --topics ["topic1","topic2"]
NAME                          SCHEMA_VERSIONS    TOPICS
avro-market-activity-value    2                  avro-market-activity
proto-market-activity-value   1                  proto-market-activity
new-subject                   0                  topic1,topic2

5. Add an additional existing topic to a subject:

> rpk registry subject edit new-subject --topics ["topic1", "topic2", "topic3"]
NAME                          SCHEMA_VERSIONS    TOPICS
avro-market-activity-value    2                  avro-market-activity
proto-market-activity-value   1                  proto-market-activity
new-subject                   0                  topic1, topic2, topic3

[emaxerrno]

this is a GREAT idea!!!!! 🚀

cc: @vsaraswat @sameers361 @dswang

[vuldin]

Here is a PR that provides one version of this CLI: redpanda-data/redpanda-examples#20

It's not a PR for adding this capability to rpk (that will come eventually though). Instead this is just a node app that will help solidify the registry-focused arguments and output. Once people think it looks good then that can be the basis for a PR adding this functionality to the rpk CLI.

The PR is in draft mode since it builds on another PR that is waiting to be merged, but if you are interested you can check out the branch and follow the instructions in the PR description (or just run node registry help)