fix support level for HTTPRequestHeaderFilter

[hbagdi]

What type of PR is this?

/kind bug
What this PR does / why we need it:
As noted in #655, a core filter with extended support fields is
confusing and also seems to be incorrect.
This patch drops the extended field from the filter and is a partial fix
for the referenced issue.

HTTPRequestMirroFilter has not been patch as GEP-718 could potentially
simplify that.
Which issue(s) this PR fixes:

Partial fix for #655

Does this PR introduce a user-facing change?:

bugfix: To avoid confusion, fields within HTTPRequestHeaderFilter are no more marked as 'extended'. All fields must be supported by implementations for this core filter.

[k8s-ci-robot]

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[hbagdi]

/cc @banks @robscott

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[robscott]

Thanks!

/lgtm