[chore] Add validation to push-tags Makefile target

[crobert-1]

Description:
When releasing v0.150.0, I misread directions and didn't understand steps 1 and 2 were two different PRs. This resulted in needing to create a patch release v0.150.1 to address the incorrect contents of the v0.150.0 tag.

This PR moves the functionality of creating and pushing a tag to a standalone script and validates that the manifests have been properly updated by the [chore] Prepare release PR.

Testing:

$ make push-tags
TAG must be set (e.g. TAG=v0.100.0)
make: *** [push-tags] Error 1

$ make push-tags TAG=0.151.0
TAG must start with lowercase 'v' (e.g. v0.100.0)
make: *** [push-tags] Error 1

$ make push-tags TAG=v0.151.0
Version mismatch in distributions/otelcol-contrib/manifest.yaml: dist.version is set to '0.150.1', expected '0.151.0'
Please ensure the '[chore] Prepare release 0.151.0' PR has been merged.
If this version mismatch is expected, please re-run with the extra argument 'VALIDATE=false'
make: *** [push-tags] Error 1

$ make push-tags TAG=v0.150.1
Adding tag v0.150.1
Pushing tag v0.150.1

$ make push-tags TAG=v0.151.0 VALIDATE=false
Adding tag v0.151.0
Pushing tag v0.151.0

Disclosure: I used Codex and Claude to generate most of this script.

[crobert-1]

If it's preferred, we could rename VALIDATE to be more specific to its current purpose, something like VALIDATE_TAG_VERSION.