[schema] Introduce schema builder to build schema.

[sijie]

Motivation

Currently we are supporting POJO based schema in java clients.
POJO schema is only useful when the POJO is predefined. However
in applications like a CDC pipeline, POJO is no predefined, there
is no other way to define a schema.

Since we are using avro schema for schema management, this PR
is proposing a simple schema builder wrapper on avro schema builder.

Modifications

Introduce schema builder to build a record schema.

NOTES

Currently we only support primitives in defining fields in a record schema in this PR.
We will add nested types in future PRs.

Verifying this change

This change added tests and can be verified as follows:

• Add schema builder test to verify the schema build via builder is same as building from POJO

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (yes)
• The schema: (yes)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (not documented)
• If a feature is not applicable for documentation, explain why?
  • the feature is partially completed. documenting a partially completed feature will confuse people
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

[sijie]

run java8 tests

[jiazhai]

rerun java8 tests

[codelipenghui]

lgtm +1

[jiazhai]

rerun java8 tests

[merlimat]

Merged in 2.3.1 at
f458ec3