* feat(spanner): add support for Proto Columns (#6886)

* feat(spanner): Adding support for Proto Message and Proto Enum

* feat(spanner): Add license header to new files

* code refactoring and additional checks

* nit: code changes

* go.mod update to prevent failing builds

* go.mod update to prevent failing builds

* go.mod and build_samples.sh update to prevent failing builds

* revert back grpc version

* revert back changes

* Using standard Singers example, refactoring redundant code

* code and proto refactoring

* Add proto_type_fqn for Proto Message and Proto Enum

* code refactoring

* go mod tidy: go.mod and go.sum version updates

* add changes to support compatibility between Int64 and Enum & compatibility between Bytes and Proto

* Revert "go mod tidy: go.mod and go.sum version updates"

This reverts commit 484b00c.

* add Integration Tests for Proto Message, Proto Enum, compatibility tests

* code refactoring

* code refactoring

* add unit tests for nil proto types

* Add error when nil proto message or nil enum is passed

* feat(spanner): add support for handling null value in Proto columns (#6954)

* feat(spanner): add support for handling null value in Proto columns

* code refactor: get protoTypeFqn from user defined nil types

* code refactoring

* code refactoring

* code refactoring

* code refactoring

* Add tests for MarshalJSON and UnmarshalJSON methods

* refactoring test file

* code refactoring

* feat(spanner): add support for Array of Proto columns (#7014)

* feat(spanner): add support for array of proto columns

* refactoring comments and added negative test cases while reading array of protos

* change decoding logic of handling array of proto columns

* feat(spanner): add support for handling null values in array of proto columns (#7042)

* feat(spanner): add support for handling null values in Array of Proto Columns

* add comments for code readability

* nit: change in error message

* feat(spanner): Modify configuration for integration test and add license header (#7059)

* feat(spanner): update go-genproto dependency (#7066)

* feat(spanner): support for enum columns as keys, index and integration tests (#7091)

* feat(spanner): support for proto columns as primary key and tests for parameterized queries, primary key and indexes

* feat(spanner): close read-only transaction to prevent session leak

* feat(spanner): update table schema to have GPK on proto field

* feat(spanner): add proto changes to support proto columns feature

* feat(spanner): remove gen-proto dep overwrite for proto column support

* feat(spanner): remove gen-proto dep overwrite from kokoro build

* feat(spanner): reset array of proto, enum when null from database (#7176)

* feat(spanner): reset array of proto, enum when null from database

* feat(spanner): fix license header to fix vet.sh build

* feat(spanner): fix proto generated file to fix vet.sh build

* feat(spanner): organize imports to fix vet.sh build

* feat(spanner): golint changes to fix vet.sh build

* feat(spanner): add support for Proto column DDL (#7292)

* feat(spanner): add proto changes for Proto Columns DDL support

* feat(spanner): add Proto descriptor file and integration tests for Proto DDL feature

* feat(spanner): skip pg tests and code refactoring for proto ddl

* feat(spanner): rename NewDatabaseAdminRESTClient to NewDatabaseAdminClient due to visibility label issue fix

* feat(spanner): remove hardcoded cloud-devel host

* feat(spanner): remove hardcoded project id

* feat(spanner): revert autogenerated code

* feat(spanner): just to validate integration tests running

* feat(spanner): change copyright year

* feat(spanner): set project id for integration tests

* feat(spanner): use jsoniter instead of json for marshal and unmarshal

* feat(spanner): revert presubmit.sh changes

* chore(spanner): run integration tests in presubmit

* chore(spanner): revert presubmit.sh changes

* feat(spanner): fix json

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Co-authored-by: rahul2393 <irahul@google.com>