Validator: make `github` field required for `block_content` examples

[Laren-AWS]

Currently, the github field is optional for examples, but the downstream effect of this is that WRITEME omits the example from the README, which is not intentional.

For the root repo (possibly under strict mode?), we should require this field at least for block_content examples, and possibly for all examples so that they are correctly included in the READMEs.

It is a valid case to omit this field when the source examples are in a tributary that's stored somewhere inaccessible, so we must allow a switch for this to optional.

[Laren-AWS]

There are quite a few of these in cross_metadata.yaml, so this task will require first being implemented as a warning, then cleaning up all the metadata, then making it an actual blocking error.

[Laren-AWS]

I have the code for this update to the tool ready in a branch. I'll open an epic to track updating the example metadata.

[Laren-AWS]

Blocked until the epic to make the underlying metadata updates is completed: awsdocs/aws-doc-sdk-examples#7012.