uhv: codec honor legacy header validation configuration

[ameily]

As part of the unified header validation component, each HTTP codec will need to honor the header validation policy configuration and use the legacy parsing/validation when the policy is configured. Each codec will need to be updated to check the configured policy and either use or skip the legacy validations. Additionally, the policy will determine how the underlying library is configured (see #19750 and #19751).

For example, the h1 codec will need to check the validation policy when parsing header values and only validate the header value when operating in legacy mode.

This will need to be completed after the HCM configuration is updated to include the header validation policy, #19754.

[github-actions]

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