SignedExchange: Move StructuredHeaderParser to separate file

This will be used to parse Variants / Variant-Key header too.

Bug: 923229
Change-Id: I69e90e977a1838a942aaeb59d58c7142da8d43f4
Reviewed-on: https://chromium-review.googlesource.com/c/1415062
Commit-Queue: Kunihiko Sakamoto <[email protected]>
Reviewed-by: Tsuyoshi Horo <[email protected]>
Reviewed-by: Kouhei Ueno <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#624044}
4 files changed