Provide me with a machine parsable version in the vendir.lock.yml when githubRelease is used

**Describe the problem/challenge you have**
I really love the `tagSelection` capability in vendir for `githubRelease`, it's the best and support for conditionals is really stellar. I've started using it instead of `tag` after playing around with it for `imgpkgBundle`. For `imgpkgBundle` I'll get what the semver resolved to in the lock file:

```
apiVersion: vendir.k14s.io/v1alpha1
directories:
- contents:
- image:
tag: v1.4.0
url: projects.registry.vmware.com/tkg/tanzu_core/provider/provider-templates@sha256:6f48690e6e7ff5c81443134581cb4c80320767b345b347ef83038c74cc0ec549
path: .
path: bundle/config/upstream
- contents:
- directory: {}
path: .
path: bundle/config/overlay
kind: LockConfig

```

But then with `githubRelease` I don't get anything that is easily machine parsable for follow-on scripts:

```
apiVersion: vendir.k14s.io/v1alpha1
directories:
- contents:
- githubRelease:
url: https://api.github.com/repos/kubernetes-sigs/cluster-api-provider-aws/releases/47009918
path: .
path: bundle/config/upstream
- contents:
- directory: {}
path: .
path: bundle/config/overlay
kind: LockConfig
```

**Describe the solution you'd like**
I'm telling vendir what to check against, like this example:
```
apiVersion: vendir.k14s.io/v1alpha1
kind: Config
minimumRequiredVersion: 0.12.0
directories:
- path: bundle/config/upstream
contents:
- path: .
githubRelease:
slug: kubernetes-sigs/cluster-api-provider-aws
tagSelection:
semver:
constraints: ">0.6.5 <0.7.0"
assetNames: ["infrastructure-components.yaml"]
disableAutoChecksumValidation: true
- path: bundle/config/overlay
contents:
- path: .
directory:
path: manifests
```
I want to know what release I ended up with in such a way I can just run through:

```
yq eval '.. | select(has("tag")).tag' vendir.lock.yml vendir.yml
```

to get the tag that was resolved.

I am doing this already with `imgpkgBundle` and `githubRelease` when I explicitly set the tag, but this capability stops working if I move to `tagSelection`. I can do something similar with `helmChart` (just change to version instead of tag)

**Anything else you would like to add:**
I am happy to change up my process if there's a more robust solution here, maybe some kind of aggregate "version collector" or similar for vendir that can help establish a "lock versioning" based on what was fetched.

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by tracing how githubRelease writes vendir.lock.yml, then compare that output with the existing imgpkgBundle and helmChart lock entries described in the issue. The change is done when a tag selected by tagSelection is recorded in a machine-parsable lock entry and the provided yq query can retrieve it.

Written by the indexing model from the issue text.