fix: /inserts-stream endpoint now accepts complex types

[vcrfxia]

Description

Three changes in this PR:

(1) Prior to this PR, inserting a row with a complex column type (array, map, or struct) via /inserts-stream would fail since the SqlValueCoercer can't handle JsonArray or JsonObject. This PR fixes the bug.

(2) This PR adds server-side and client-side test coverage for complex types including decimal, array, and map. A follow-up PR will add coverage for struct as well.

(3) Besides (1), the only other non-test change in this PR is in response to #5448 (comment): rather than serializing BigDecimal as double on the client, we instead serialize BigDecimal as string in order to avoid loss of precision. The drawback is we now have an inconsistency where the decimal type is treated differently in terms of allowable casts, compared to the other numeric types. For example:

final KsqlObject obj = new KsqlObject().put("f1", new BigDecimal("1.13")).put("f2", 12.23);
System.out.println(obj.getString("f1")); // This succeeds
System.out.println(obj.getString("f2")); // This fails with "java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.CharSequence"

Testing done

Tests pass.

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 #")

[purplefox]

LGTM