.github: update codeql and linter workflows

caddy: go mod tidy

go.mod: update grpc library and deps

.github: run govulncheck over caddy module

.github: replace dependabot with daily test and govulncheck

go.mod: update deps with vulnerabilities

.github: add workflow_dispatch trigger for tests

docs: reformat plugin-design.md for semantic line breaks

allow customizing response headers passed to client

.gitignore: only ignore top-level imageproxy file

Add a metric "http_requests_in_flight"

Limit number of running transformation threads

cmd/imageproxy: add support for listening on Unix socket

dependabot: security updates only, grouped in a single PR

go.mod: target package version bumps