feat(bigquery): support IAM conditions in datasets

[shollyman]

This PR adds support for IAM conditions via the existing dataset access mechanism. To do so, the following changes are necessary:

• add the Expr type for expressing conditions, and wire it into the existing DatasetAccessEntry.
• add an option pattern to the Dataset-related RPC methods
• Add a new WithAccessPolicyVersion option for setting access policies

To expose the new functionality, this PR adds CreateWithOptions, UpdateWithOptions, MetadataWithOptions methods on Dataset that accept the new option.

[alvarowolfx]

I wonder if we can check inside the Create/Update/Metadata methods if the dataset has any AccessEntry with a non nil Condition and automatically add the AccessPolicyVersion = 3, without requiring to add the new *WithOptions methods and DatasetOption for now.

One thing that I'm concerned is if users starts to use IAM conditions and have a mixed set of Datasets with and without conditions, they have to change all of their code to adopt the new *WithOptions methods and check by themselves if the dataset has conditions or not to add the proper WithAccessPolicyVersion version. Calling the Metadata call might be the most common one that would cause more confusions.

Maybe the Dataset struct has a method like hasConditions where if checks for AccessEntry.Condition and on the Create/Update/Metadata methods we check that and add the version.

[alvarowolfx]

nit: missing space here after //

[shollyman]

Resolved.

[alvarowolfx]

per internal discussion, it's better for users to be explicit about integrating with IAM conditions, so makes sense to have the *WIthOptions methods. When they start adopting this feature, they can use Policy version 3 for all of they dataset access usage.