Support defining and validating schema on Python client

[merlimat]

Motivation

Allow applications to declare the schema for messages in python client library.

The schema is used to perform validation against the topic schema and also
for serialization/deserialization

Added support for Bytes, String, JSON and Avro definitions.

The schema of a message can be easily defined as:

import pulsar.schema

class Example(pulsar.schema.Record):
    a = Integer()
    b = String()
    c = Long(default=5)

With that, one can create a producer/consumer. Eg:

producer = client.create_producer(
                    topic='my-topic',
                    schema=pulsar.schema.AvroSchema(Example)

producer.send(Example(a=1, b='hello'))

Type is also validated to ensure a producer will only send data of the same type.

TODOs (in separate PRs):

• Add tests for schema compatibility across Python and Java
• Documentation and examples
• AUTO_CONSUME schema type

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run integration tests

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run integration tests

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run integration tests

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run integration tests

[sijie]

run integration tests

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run integration tests

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run integration tests

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run integration tests

[sijie]

run integration tests

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run java8 tests