Schema specification on messages produce

[iliax]

Current behavior:

When we produce messages curren serialization logic looks like

1. receive input message (key+value) as a text
2. search for key & value schemas using TopicNameStrategy (-key, -value postfixes)
3. if schema(s) are found - we try to serialize key/value using it and throw error if it is not possible
4. if shema(s) not found - we treat input k/v as a string.

---

Problems:

1. schema search is done implicitly - we don't show in UI what schema will be used for serialization (of no schema will be used). It can cause problems for consumers and can break whobe downstream processing.
2. if user is using RecodNameStrategy or TopicRecordNameStrategy for subject naming he can't explicitly set desired subject name

---

Suggestions:
In produce page we should:

1. show subject that will be used for serializing both key and value. It can be a dropdown near key & value input area
2. by default we should show "Raw String"/ "No Schema" if no subject found for k/v using TopicNameStrategy. Or corresponding subject names found by TopicNameStrategy.
3. User should be able to override found subjects. (ex. using dropdown list with embedded search field).
4. Also, it can be useful if we will allow to chose version of chosen schema

CC @germanosin @Haarolean