Switch KSQL over from using the Connect Schema type to represent schemas to KsqlSchema

[big-andy-coates]

Description

A preparatory PR for the structured keys work (#824) that switch KSQL over from internally passing around and storing schemas using the Connect Schema type to a KSQL specific KsqlSchema.

Some places, e.g. serde classes, still use the Schema type as they are dealing specifically with the value only so don't need the key.

Internally, KsqlSchema still uses the Connect Schema type. Later it will be enhanced to have the key schema as well.

The PR tries to minimise code change as this is already a wide reaching PR. The main changes are:

1. Switch KSQL from using Schema to KsqlSchema in most places in the code.
2. Moved code from SchemaUtil into KsqlSchema where it made sense. Note: this PR does not attempt to fix bugs or refactor the implementation of this methods. That will come later.
3. A sneaky rename of two params to JoinNode that got missed on a previous PR.

Testing done

Lots more tests added around the SchemaUtil code that now lives in KsqlSchema.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[agavra]

I did a quick scan and it lgtm - one comment about json backwards compatibility but the approach is sound to me (barring green build of course).