Custom Responses based on upstream reply

[pradeepcrao]

Title: Provide the ability to customise upstream responses

Description:
The ability to customise responses from the upstream (particularly errors) allows Envoy to provide customised web pages to the user which is particularly useful in multi-tenant environments where the default page might be perceived as jarring to the end user. Some functionality does currently exist using LocalReplyConfig, but this has the following limitations:

• The custom responses are always locally defined. This can lead to memory bloat issues in multi-tenant environments.
• ResponseMappers allow for only one filter per custom response.
• Advanced matching/filtering logic is not easy to specify.
• One cannot customise 2xx responses.

This proposal aims to solve the above limitations by defining a new filter to customise upstream responses.

[pradeepcrao]

/assign @pradeepcrao

[htuch]

#11007 for LocalReplyConfig introduction reference. CC @mattklein123 @qiwzhang

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[github-actions]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted" or "no stalebot". Thank you for your contributions.