feat: Update Datastore to V2

[Hectorhammett]

This is an effort to update the Datastore library to start using a standardized way. This consists in removing the ConnectionInterface and replacing it for our generated client.

b/338660654

BREAKING_CHANGE_REASON=removing connection classes in favor of generated client, which will be breaking. We are incrementing a new major version

[bshaffer]

Second pass (all minor stuff)

[bshaffer]

Looks great! Just a few suggestions for options validation (not required), and also please update the PHPDoc for $options in Operation::allocateIds to list the supported options

[bshaffer]

I think we should go through each handwritten class file in src and promote the properties to the __construct constructor and give them types.

[bshaffer]

the third option here doesn't exist! That's only required for pluck, not pluckArray.
also, we can format this to be a bit less verbose (IMHO)

Suggested change

	$readOptions = $this->createReadOptions($this->pluckArray(
	[
	'readConsistency',
	'transaction',
	'readTime'
	],
	$options,
	false
	));
	$readOptions = $this->createReadOptions($this->pluckArray(
	['readConsistency', 'transaction', 'readTime'],
	$options,
	));

[bshaffer]

For these we could use inline constructor properties:

    public function __construct(
        private string $projectId,
        private bool $encode,
        private bool $returnInt64AsObject,
    ) {

[bshaffer]

here as well - all of these arguments can be defined as properties in the constructor:

    public function __construct(
        private DatastoreClient $gapicClient,
        private string $projectId,
        private string $namespaceId,
        private EntityMapper $entityMapper,
        private string $databaseId = ''
    ) {

[bshaffer]

both $entityMapper and $query can be given types and defined in the constructor

[bshaffer]

properties $operation, $transactionId, and $projectId should be typed and defined in the constructor