Allow configurable downgrade of ETag validator strength

[awwong1]

This PR allows users to override the default strong ETag validator behaviour to use weak ETag validators.

I was encountering issues where my worker's returned strong ETags were automatically converted to weak ETags by Cloudflare. This change allows the developer to use weak ETags and preserve 304 responses (e.g. on *.workers.dev domains).

The alternative to this is specifying page rules to preserve strong ETags (possibly enterprise only)

• Change has unit tests
• Readme has been updated with documentation and usage
• This is a backwards compatible change
• I have tested this on my own Cloudflare workers

#311

[awwong1]

Are there any additional steps you need me to do before this PR can be accepted?
CC: @codewithkristian