Inherit `provider.tags` onto the AppSync API

Split out from the v2 RFC (#378 thread, raised by @tuukkafc).

Problem

Tags defined at the Serverless provider level are not applied to the AppSync API. Users
expect provider tags to propagate to resources the way they do for other resources, but the
plugin only uses tags defined under appSync.tags.

Current state (v2)

Api.getTagsConfig() reads only this.config.tags (the appSync.tags block) and there is
no reference to provider.tags (or provider.stackTags) anywhere in the plugin. So
provider-level tags are dropped for the AppSync API.

Proposal

Merge provider.tags with appSync.tags when building the API's Tags, with appSync.tags
taking precedence on key conflicts. Small, surgical change in getTagsConfig().

Scope notes

• AWS::AppSync::GraphQLApi supports Tags; AWS::AppSync::DataSource and
  AWS::AppSync::Resolver do not, so the practical scope is the API resource (and
  potentially the log group, which is a separate AWS::Logs::LogGroup).
• Worth deciding whether to also fold in provider.stackTags.