Creates a spannerlib module and adds functions for CreatePool and CreateConnection.

Adds a Java wrapper for SpannerLib and some simple tests for this wrapper.

Adds an Execute function for SpannerLib that can be used to execute any type
of SQL statement. The return type is always a Rows object. The Rows object
is empty for DDL statements, it only contains ResultSetStats for DML statements
without a THEN RETURN clause, and it contains actual row data for queries and
DML statements with a THEN RETURN clause.

The Execute function can also be used to execute client-side SQL statements,
like BEGIN, COMMIT, SET, SHOW, etc.

Adds an ExecuteBatch function to SpannerLib that supports executing DML or DDL
statements as a single batch. The function accepts an ExecuteBatchDml request
for both types of batches. The type of batch that is actually being executed
is determined based on the statements in the batch. Mixing DML and DDL in the
same batch is not supported. Queries are also not supported in batches.

Adds a WriteMutations function for SpannerLib. This function can be used
to write mutations to Spanner in two ways:
1. In a transaction: The mutations are buffered in the current read/write transaction.
   The returned message is empty.
2. Outside a transaction: The mutations are written to Spanner directly in a new read/write
   transaction. The returned message contains the CommitResponse.

Adds a .NET wrapper and tests for SpannerLib.

Adds a gRPC server that exposes the API of SpannerLib. This allows clients
to connect to run SpannerLib as a child process and connect to it using
gRPC.

* chore: add Java wrapper for SpannerLib

Adds a Java wrapper for SpannerLib and some simple tests for this wrapper.

* chore: add Execute function for SpannerLib (#529)

* chore: add Execute function for SpannerLib

Adds an Execute function for SpannerLib that can be used to execute any type
of SQL statement. The return type is always a Rows object. The Rows object
is empty for DDL statements, it only contains ResultSetStats for DML statements
without a THEN RETURN clause, and it contains actual row data for queries and
DML statements with a THEN RETURN clause.

The Execute function can also be used to execute client-side SQL statements,
like BEGIN, COMMIT, SET, SHOW, etc.

* chore: add transaction support for SpannerLib (#530)

* chore: add transaction support for SpannerLib

* chore: add ExecuteBatch to SpannerLib (#531)

* chore: add ExecuteBatch to SpannerLib

Adds an ExecuteBatch function to SpannerLib that supports executing DML or DDL
statements as a single batch. The function accepts an ExecuteBatchDml request
for both types of batches. The type of batch that is actually being executed
is determined based on the statements in the batch. Mixing DML and DDL in the
same batch is not supported. Queries are also not supported in batches.

* chore: add WriteMutations function for SpannerLib (#532)

Adds a WriteMutations function for SpannerLib. This function can be used
to write mutations to Spanner in two ways:
1. In a transaction: The mutations are buffered in the current read/write transaction.
   The returned message is empty.
2. Outside a transaction: The mutations are written to Spanner directly in a new read/write
   transaction. The returned message contains the CommitResponse.

…b-dotnet-wrapper

Adds a gRPC wrapper for .NET. This wrapper uses the gRPC API to connect to SpannerLib.
The gRPC channel uses a Unix domain socket.

Adds various build scripts for the .NET wrapper.