This commit adds weak ETag support in ShallowEtagHeaderFilter.
This improves the behavior of the filter in tow ways:
* weak ETags in request headers such as `W/"0badc0ffee"` will be
compared with a "weak comparison" (matching both weak and strong ETags
of the same value)
* when enabled with the "writeWeakETag" init param, the filter will
write weak Etags in its HTTP responses
Issue: SPR-13778
1c2ac49f2a