Make Headers Setter context default value configurable (#35151)

[kyo-ke]

Description:
Headers Setter extension

To make deafault value of Headers Setter extension's context, this PR adds DefaultValue option to HeaderConfig.

Link to tracking Issue:
Resolves #35151
also resolve #34412 (find this after creating above issue)

Testing:
add unittest to extension/headerssetterextension/extension_test.go.
testcases are for both with default_value option and without default_value.
confirm not changing behaviour with this PR and new functionality is forking.
also integration test with below configulation is done locally

  headers_setter:
      - action: upserte
        key: X-Scope-OrgID
        from_context: X-Scope-OrgID
        default_value: fake

Documentation:

default value can be added via default_value option.

Co-authored-by: So Koide so.koide@morganstanley.com

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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: kyo-ke / name: kyosuke (7dcde09, 10b941f, 650620a, e947dc5, bba54d2)

[jpkrohling]

Closing this in favor of #34455