[improve][client] PIP-420: Supports users implement external schemas

[gaoran10]

Motivation

This is the implementation for PIP-420.

Modifications

Add a new SchemaType EXTERNAL and modify the corresponding protobuf messages.
Add a new method Map<String, String> getConfigs() for interface SchemaInfoProvider.
Add a new configuration schemaProperties for the Pulsar client builder.
Add ExternalSchemaCompatibilityCheck for the broker configuration files broker.conf and standalone.conf.
Close the schema while closing the Producer and Consumer.

Verifying this change

Add a test to verify the Schema interface changes.

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

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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