[MBUILDCACHE-92] Allow to define 'reconciles' at plugin execution level

**[Réda Housni Alaoui](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=ralaoui)** opened **[MBUILDCACHE-92](https://issues.apache.org/jira/browse/MBUILDCACHE-92?redirect=false)** and commented

I have the following case:

```xml

com.github.eirslett
frontend-maven-plugin


yarn-build-dev

yarn

prepare-package

${build-dev.skip}
build:dev



yarn-build-prod

yarn

prepare-package

${build-prod.skip}
build:prod

```

`yarn-build-prod` produces a more complete output than `yarn-build-dev` but it also takes more time to execute:
- on a dev environment (build-prod.skip=true and build-dev.skip=false) using a cache produced with build-prod.skip=false and build-dev.skip=true should lead to a cache hit
- on a CI environment (build-prod.skip=false and build-dev.skip=true) using a cache produced with build-prod.skip=true and build-dev.skip=false should lead to a cache miss

The only way I can think of implementing this is to track property `skip` only for execution id `yarn-build-prod`, but not for execution id `yarn-build-dev`.

But I can't, because as of now, this extension allows to track plugin property with reconcile at plugin goal level, not execution level.

---

**Affects:** 1.1.0

No files or tests are named. Start by tracing the existing plugin-goal reconcile handling and how execution IDs and the skip property are represented; the change is complete when reconcile configuration can distinguish yarn-build-prod from yarn-build-dev and produce the described cache hit and miss outcomes.

Written by the indexing model from the issue text.