beam:coder:row:v1 implementations should respect encoding_position

### Problem/Status
The schema proto has an [encoding_position field](https://github.com/apache/beam/blob/2c619c81082839e054f16efee9311b9f74b6e436/model/pipeline/src/main/proto/schema.proto#L55) that is currently unused in every row coder implementation. The intention of this field is that it indicates an alternative order for the fields to be encoded in by [beam:coder:row:v1 implementations](https://github.com/apache/beam/blob/1e60f383fb39b9ff8d44edcbe5357da4c1e52378/model/pipeline/src/main/proto/beam_runner_api.proto#L937-L990). Currently all the implementations ignore this field, and always encode the fields in the order that they appear in the schema.

### Motivation
The idea with the encoding position is that it will give runners a way to enforce schema compatibility (BEAM-9502), by re-ordering the way fields are encoded when the schema changes between two job submissions. Schema changes could be due to fields re-ordering, or field additions/deletions.

### Code pointers
The Python beam:coder:row:v1 implementation lives in [row_coder.py](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/coders/row_coder.py)
The Java implementation is a little more complicated, distributed between [SchemaCoder](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/SchemaCoder.java), [RowCoder](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/coders/RowCoder.java), and [RowCoderGenerator](https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/coders/RowCoderGenerator.java). RowCoderGenerator contains the code relevant to this jira - it uses bytebuddy to generate Java code for the coder. We need it to generate code that puts fields in the order specified by encoding_position.

### Testing
Python and Java implementations should be tested with unit tests ([RowCoderTest](https://github.com/apache/beam/blob/master/sdks/java/core/src/test/java/org/apache/beam/sdk/coders/RowCoderTest.java), [row_coder_test](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/coders/row_coder_test.py)). We should also test them for compatibility by adding test cases that exercise the encoding_position in [standard_coders.yaml](https://github.com/apache/beam/blob/master/model/fn-execution/src/main/resources/org/apache/beam/model/fnexecution/v1/standard_coders.yaml). These tests will be executed by [CommonCoderTest](https://github.com/apache/beam/blob/master/runners/core-construction-java/src/test/java/org/apache/beam/runners/core/construction/CommonCoderTest.java) and [standard_coders_test](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/coders/standard_coders_test.py). There's some example code for generating a new test case [here](https://github.com/apache/beam/blob/master/model/fn-execution/src/main/resources/org/apache/beam/model/fnexecution/v1/standard_coders.yaml#L387-L400).

Imported from Jira [BEAM-10277](https://issues.apache.org/jira/browse/BEAM-10277). Original Jira may contain additional context.
Reported by: bhulette.