feat!: Spanner V2

[bshaffer]

TODO

• Depends on feat!: owlbot updates for Spanner V2 #7840
• Depends on cl/722693790
• Verify that configureTransactionOptions is either no longer being called twice in runTransaction, or that it has a good reason for being called twice (cc @purva9413 ) moved to chore(Spanner): TransactionOptionsBuilder refactor (WIP) #8653

BREAKING_CHANGE_REASON=Spanner v2

BEGIN_VERSION_OVERRIDE
Core: 1.68.0
END_VERSION_OVERRIDE

[purva9413]

I am unusure of the call to configure the txn options here. If the $options['transactionOptions'] is set, this is essentially a no-op, and if this is not set it just initializes the readWrite options on the transactionOptions. We could add in a method to just init readWrite options (I have a similar suggestion in TransactionConfigurationTrait.php) and then simplify this with something like

$options['transactionOptions'] = $options['transactionOptions'] ?? $this->initReadWriteTransaction(); 

[purva9413]

This will also help untangle some of the calling stack as there is a similar call to configure here as well.

Happy to discuss this in more details, I think our unit tests should be able to catch issues if they come up with this change.

[bshaffer]

We've added calls using validateOptions to at least add structure to these functions.

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