* Mixer filter fixes

* Address review comments

* Changes to listener build, revert OnListener APIs

* Move buildListener just before callbacks

* Address review comments

* Address review comments #2

* Address comments #3

* Address review comments #4

* Change return to struct value

* # This is a combination of 5 commits.
# This is the 1st commit message:

update istio.io/api for new mcp definitions

# This is the commit message #1:

update mcp server

# This is the commit message #2:

update snapshot package

# This is the commit message #3:

use mcp service in galley/pkg/server

# This is the commit message #4:

fix linter

* update istio.io/api for new mcp definitions

update the latest mcp service definitions from istio.io/api

* fix linter error (again?)

*  Separate generated metadata into their own, top-level package: pkg/metadata

- Introduce builder pattern to the core schema types that are used by pkg/metadata,
  to ensure runtime immutability after build.
- Move the Kubernetes related machinery to their own package, pkg/kube/source, to
  separate it from core Kubernetes related abstractions and utility code.
- Bolster tests and cleanup tests accordingly.

* Add builder support to InMemory snapshot. (#3)

* Add builder support to InMemory snapshot.

* Clone resurces as well.

* Use proto.Clone, instead of manual copying.

* Add a testing server for MCP. (#4)

* Add a testing server for MCP.

* Add overridable port field.

* Don't parse the port number.

* Minor bug & linter fix (#6)

* Fix a bug in the in-memory source implementation where the item itself
was missing when publishing already-existing items during the initial
sync phase.

* Fix lint issue.

* Updates to acommodate merge.

* Update Gokg.lock.

* Reflect utilities