a2a: Initial implementation of a2a filter

[tyxia]

Initial implementation: filter/config skeleton, request header handling. request body will be handled in the next PR.

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #43602 was opened by tyxia.

see: more, trace.

[tyxia]

/coverage

[repokitteh-read-only]

Coverage for this Pull Request will be rendered here:

https://storage.googleapis.com/envoy-cncf-pr/43602/coverage/index.html

For comparison, current coverage on main branch is here:

https://storage.googleapis.com/envoy-cncf-postsubmit/main/coverage/index.html

The coverage results are (re-)rendered each time the CI Envoy/Checks (coverage) job completes.

🐱

Caused by: a #43602 (comment) was created by @tyxia.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/coverage-shephards: FYI only for changes made to (test/coverage.yaml).
envoyproxy/coverage-shephards assignee is @RyanTheOptimist
CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #43602 was ready_for_review by tyxia.

see: more, trace.

[tyxia]

/retest

[tyxia]

/retest