Add support for ECS Anywhere

[tuxpower]

What does this PR do?

Add support for self-managed on-premise infrastructure with ECS Anywhere.

Motivation

Extend ECS provider.

Fixes #8795

More

• Added/updated tests
• Added/updated documentation

Additional Notes

make validate fails with the following error:

docker run -v "/var/run/docker.sock:/var/run/docker.sock"   -e OS_ARCH_ARG -e OS_PLATFORM_ARG -e TESTFLAGS -e VERBOSE -e VERSION -e CODENAME -e TESTDIRS -e CI -e CONTAINER=DOCKER		 -v "/home/jgaspar/Documents/repos/traefik/dist:/go/src/github.com/traefik/traefik/dist" "traefik-dev:feature-ecs-anywhere" ./script/make.sh generate validate-lint validate-misspell validate-vendor
---> Making bundle: generate (in /go/src/github.com/traefik/traefik/script)

---> Making bundle: validate-lint (in /go/src/github.com/traefik/traefik/script)
pkg/provider/ecs/ecs.go:210: Function 'listInstances' is too long (249 > 230) (funlen)
func (p *Provider) listInstances(ctx context.Context, client *awsClient) ([]ecsInstance, error) {
make: *** [Makefile:139: validate] Error 1

That's because I had to duplicate some of the code for managed instances ending up on a much bigger function.

[tuxpower]

This PR replaces #9127 as requested by @mmatur.
#9127 (comment)

[mmatur]

LGTM 👏🏼 🎉

[mmatur]

LGTM 👏🏼 🎉

[rtribotte]

LGTM 👍

[kevinpollet]

LGTM