Allow extensions to bind to multiple revisions

Upstreams like IETF and OpenConfig a revising their models, which means the same extension can appear with multiple revisions.

While we can specify the extension support to be bound to no revision, this is not accurate and does not completely cover the range of possibilities allowed for RFC7950 – a model does not have to specify a revision initially and may specify revision later. In this context a non-present revision means that the extension should be binding to non-revisioned model only.

To properly support this use case, StatementDefinition needs to express whether statementName() is specifying exact revision, any wildcard, or a range of revisions.

Range of revisions is required to support the case where a semantic change occurs in the model and there are multiple revision cover one semantic. An example is: what would happen if openconfig-extensions.yang defined openconfig-encrypted-value in two revisions before switching to openconfig-hashed-value?