feat: support for Table ACLs

[shollyman]

This PR adds support for table-level permissions to BigQuery, which is implemented via the IAM meta API.
There's three mechanisms exposed: getIamPolicy, setIamPolicy, and testIamPermissions.

The service exposes a service-centric version of the responses, so we include a policy converter to map to/from the more general cloud IAM messages.

Differences from the other veneer (java-storage):

• changed the response format of TestIamPermissions. The general pattern is to pass in a resource (table) and a list of permissions to check, and it returns the subset of that permission list which the caller actually holds on the resource. The storage veneer chose to implement this as a list of bools instead, which is nonstandard.

• Policy conversion is much more forgiving. In java-storage, it's only top-level resources which return policies, so there's an expectation that all responses have actual bindings. Tables in BQ are nested within other elements that also convey permissions, so it's far more common and likely to have empty bindings on policies.

[codecov]

Codecov Report

Merging #653 into master will decrease coverage by 0.42%.
The diff coverage is 57.27%.

@@             Coverage Diff              @@
##             master     #653      +/-   ##
============================================
- Coverage     81.49%   81.07%   -0.43%     
- Complexity     1226     1236      +10     
============================================
  Files            77       78       +1     
  Lines          6220     6329     +109     
  Branches        691      702      +11     
============================================
+ Hits           5069     5131      +62     
- Misses          792      833      +41     
- Partials        359      365       +6     

Impacted Files	Coverage Δ	Complexity Δ
...y/src/main/java/com/google/cloud/bigquery/Acl.java	71.42% <ø> (ø)	13.00 <0.00> (ø)
.../main/java/com/google/cloud/bigquery/BigQuery.java	76.23% <0.00%> (-1.15%)	0.00 <0.00> (ø)
...ava/com/google/cloud/bigquery/BigQueryOptions.java	88.88% <ø> (ø)	11.00 <0.00> (ø)
.../google/cloud/bigquery/spi/v2/HttpBigQueryRpc.java	4.94% <0.00%> (-0.26%)	2.00 <0.00> (ø)
...n/java/com/google/cloud/bigquery/PolicyHelper.java	65.00% <65.00%> (ø)	6.00 <6.00> (?)
...n/java/com/google/cloud/bigquery/BigQueryImpl.java	82.84% <73.33%> (-0.75%)	77.00 <3.00> (+3.00)	⬇️
...c/main/java/com/google/cloud/bigquery/TableId.java	85.18% <100.00%> (+1.18%)	15.00 <1.00> (+1.00)
.../com/google/cloud/bigquery/spi/v2/BigQueryRpc.java	82.60% <100.00%> (+0.79%)	0.00 <0.00> (ø)

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[pmakani]

LGTM!, except clir and codecov.