cdn: add sign_cookie example with tests

[mpwarres]

This PR expands the existing snippet for Cloud CDN Signed URLs to also illustrate Signed Cookies, a feature currently in closed alpha. This feature extends Cloud CDN's Signed URL functionality.

[mpwarres]

@elithrar FYI

[kurtisvg]

If this a sample for a closed alpha, why are we putting it in a public github repo?

Additionally, if you team owns these samples, please add yourself in the CODEOWNERs file.

[elithrar]

@kurtisvg This will go GA shortly (within a week) and we are attempting to sync this up with the need to be in the public docs for launch day.

Is there a better process to handle cases like this? We are not concerned about folks seeing this PR ahead of being merged.

[kurtisvg]

@elithrar Most folks use GOB to stage samples when not publicly available, but if you aren't concerned about public knowledge this is fine.

We won't be able to merge it into master until the feature is publicly available, but you can ping me when that happens and I'll take care of it.

[mpwarres]

PTAL again:

• Added code example and tests for Signed URL prefix.
• Updated CODEOWNERS entry for /cdn/
• Updated region tags.

Thanks!

[tmatsuo]

@mpwarres

In case you don't have access to the build logs. Here is the lint output:

./snippets.py:182:5: F821 undefined name 'sign_cookie_parser'
    sign_cookie_parser.add_argument(
    ^

It seems like sign_cookie_parser is defined way below.

[tmatsuo]

JFYI, from python 3.6, you can use f-string

Then it will be

policy = (
    f'URLPrefix={encoded_url_prefix}:Expires={expiration_timestamp}:'
    f'KeyName={key_name}'
)

[mpwarres]

TIL--thanks!